Signal from turned-off phone? - Page 1
Forum section: electronics / general

author: u666sa
author_rank: Frequent Contributor
timestamp: October 12, 2025, 10:17:47 am
content: Can turned-off smart phone be tracked using special equipment? Let's assume device is turned off by user, or the battery reached 0% and it turned off automatically. Let's assume it is either iPhone or an Android. Let's assume user went into the woods 12 days ago, out of reach of cell towers, got lost, phone turned off because of discharge or user turned it off himself. Is there special magic packet or signal that can be sent to wake up the device and make it talk?

author: Sorama
author_rank: Frequent Contributor
timestamp: October 12, 2025, 10:32:46 am
content: We really need to explain the necessity of power??

author: kite31
author_rank: Regular Contributor
timestamp: October 12, 2025, 10:50:12 am
content: The magic packet is called a battery which you signal with a charge current.I can understand the query to the extent that a phone switched off has non-volatile memory for when it is switched on again, or it is quiescent with background functions when in a sleep state, but off (shut down) or no battery mean no current function.edit:clarify

author: u666sa
author_rank: Frequent Contributor
timestamp: October 12, 2025, 10:55:28 am
content: I'm asking because a family of 3 got lost in the woods about 12 days ago in the far east Siberia. Authorities tracked signal from aswitched off device, using special equipment. Right now search is conducted in the place where this signal was located, prior that quadrant wasn't searched.Translation of paragraph 2: "Investigators reported that Sergei Usoltsev's cell phone was located seven kilometers from the village of Kuturchin in the Partizansky District.The signal was from a switched-off phone. Using specialized equipment, its approximate location was determined."https://t.me/s/shot_shot/87679https://t.me/s/shot_shot/87812It is unknown if phone was switched off manually or discharged. The article clearly states that signal was received using special equipement from a turned-off device. We know that 12 days have passed since they went missing and this special equipement was used.

author: kite31
author_rank: Regular Contributor
timestamp: October 12, 2025, 11:03:05 am
content: Does "switched off" mean actually off or a sleep state, is my question. The latter will wake to signal occasionally while battery in that state can last a month I think.Other speculative queries are: given the distance is only 7 kilometers, could they not have triangulated its approximate location from prior tower records? Or, a transmission which the phone antenna passively reflects, like a mirror, may exist (but I haven't a clue about that).It is a press report; I know from experience that reporters are easily confused, especially about remarkable events.

author: Psi
author_rank: Super Contributor
timestamp: October 12, 2025, 11:16:27 am
content: Who really knows what goes on in the bios firmware of a cellphone when the voltage is too low to boot up the OS.Might it ping the tower periodically, sure, it's possible. Does it, I have no idea.

author: Halcyon
author_rank: Unknown
timestamp: October 12, 2025, 11:48:18 am
content: Quote from: u666sa on October 12, 2025, 10:17:47 amSpeaking from my experience in both law enforcement (including from the covert side), and digital forensics. No. Most (all) modern smart phones do not, nor have the capability to wake up or ping some kind of signal from a purely wireless interface, when shut down. It's definitely not a method we used when an individual was being targeted.That being said, turning your phone off alone won't make you "invisible" if you're up to no good.

author: Sorama
author_rank: Frequent Contributor
timestamp: October 12, 2025, 01:33:47 pm
content: What the heck…If there is no power, there is no power.Back to basics for some ?

author: Bud
author_rank: Super Contributor
timestamp: October 12, 2025, 02:06:26 pm
content: The linked post does not say a signal was Detected using special equipment, it says its location was Determined using special equipment, which may refer to analysis of data prior to the phone went off. Open to interpretation.

author: radiolistener
author_rank: Super Contributor
timestamp: October 12, 2025, 02:30:32 pm
content: If the battery is completely drained to 0 V, the phone cannot transmit or respond in any way — there’s simply no power available. In that case, locating it is only possible by passive means, such as using a metal detector or, in theory with active radar, detecting resonance from antennas or internal components — but that would be extremely difficult beyond a very short range.If the battery still has some charge, many modern phones remain partially active even when “powered off”. For example, iPhones running iOS 15 or later can keep a low-power Bluetooth/UWB module running to periodically broadcast “Find My” beacons. If a compatible receiver picks up those beacons, the device’s approximate location can be determined.Some Android devices may have similar low-power tracking features, but they are typically not available to the public — and if they exist, such capabilities are usually restricted to law-enforcement or system-level use only. And they need charged battery. With fully discharged battery it just cannot transmit something.

author: RoGeorge
author_rank: Super Contributor
timestamp: October 12, 2025, 02:34:37 pm
content: I'll put $10 on "might be possibly for such a magic packet to exist", but only speculating here, no proof.I don't have a phone with national emergency alert system, does the emergency broadcast normally wakes up the turned off phones, too, or the alert message is received only by the running phones, no airplane mode and such?

author: oPossum
author_rank: Super Contributor
timestamp: October 12, 2025, 03:38:49 pm
content: Recent iPhone can remain active after power 'off' to facilitate finding them.

author: Bud
author_rank: Super Contributor
timestamp: October 12, 2025, 04:00:21 pm
content: This one likely rely on existance of other devices/wifi networks nearby.

author: nctnico
author_rank: Super Contributor
timestamp: October 12, 2025, 04:02:24 pm
content: Quote from: Sorama on October 12, 2025, 01:33:47 pmPhone batteries are never discharged down to last drop. There is always something left to preserve battery life. The battery charge level shown on screen has very little to do with the actual battery charge level; there is a fudge factor on top to keep the user happy. This remaining charge is more than enough to power a simple Bluetooth beacon for a very long time. Think about bluetooth tyre pressure sensors; these run for ages on a tiny button cell. All in all it is very possible to keep a beacon active while the phone is shutdown and the battery shows empty.

author: radiolistener
author_rank: Super Contributor
timestamp: October 12, 2025, 04:20:40 pm
content: You're right that most phone batteries are not allowed to drop to an absolute 0 V and that tiny, low-power radios (like Bluetooth) can run for a very long time on a small residual charge — similar to tyre-pressure sensors running from a button cell. That makes continuous beaconing plausible in some circumstances.However, a far more energy-efficient approach is duty-cycled wake-on-radio: the device keeps only a tiny low-power front end active (or periodically powers the receiver for short intervals) and otherwise stays fully quiescent. It only wakes fully and transmits when it detects a specially-addressed/authenticated packet. Because listening can be intermittent and the device transmits only on demand, this method uses orders of magnitude less energy than continuous advertising.

author: gamalot
author_rank: Super Contributor
timestamp: October 12, 2025, 04:33:44 pm
content: Quote from: Psi on October 12, 2025, 11:16:27 amOf course not, if that were the case, people would've noticed long ago.

author: rteodor
author_rank: Frequent Contributor
timestamp: October 12, 2025, 05:03:42 pm
content: Quote from: RoGeorge on October 12, 2025, 02:34:37 pmSaw it quite recently: two identical Android phones one off, one on. Only the one that was turned on went to scream.Neither one had SIM's in them. I suppose the alert is a one way channel.Software OS versions were slightly different but I do not think that matters.I plan to cripple one such phone of most of its radio functions by deleting relevant firmware files from the firmware partition. If I can get to that partition of course, otherwise I would have to deal with the antennas directly.As for the OP: lets not take the bait of psyops.LE: BTW, what the the phones in storage and on display in the shops do when receiving an alert ?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 12, 2025, 05:08:06 pm
content: Quote from: Sorama on October 12, 2025, 01:33:47 pmNothing physically wrong with the idea. Phone UI can say 0% battery and "turn off" with actual 10% state-of-charge (or 20%, or anything!), and then it can do intermittent connectivity, say, either actively connect to the usual network e.g. once an hour, or, enter listening mode periodically detecting some special transmit. It could easily do that for a week! Think about it - good old "non-smart" phones were able to be 24/7 connected for two weeks or so with much smaller battery.Do they really do that? Probably no. But the idea is completely plausible and easy to implement, doesn't break any "basics". OP should remember that mainstream media is basically 100% guaranteed to get 97% of technical details, even important ones, totally and colossally wrong, usually in most absurd ways. In this case, they could just mean that latest tower information (when the phone was still alive) was used.I believe they don't do that because if they did, it would be easy to measure and document independently. We would know about it.

author: tom66
author_rank: Super Contributor
timestamp: October 12, 2025, 05:51:44 pm
content: Some iPhones remain findable after power off, as a function that can be enabled/disabled by the user. This would only be via Bluetooth though, not the cell modem. I suppose with a highly focused antenna and a high gain, high sensitivity amplifier you might be able to pick something like that from 1-2km away, but I doubt much better than that is possible.https://discussions.apple.com/thread/254909274?sortBy=rank

author: tooki
author_rank: Super Contributor
timestamp: October 12, 2025, 06:28:44 pm
content: Quote from: oPossum on October 12, 2025, 03:38:49 pmThat opt-in feature just keeps one specific Bluetooth function going for an extra 24h. (In other words, it devolves into an AirTag for 24h.)So definitely not after 12 days!

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 12, 2025, 07:48:07 pm
content: Quote from: tom66 on October 12, 2025, 05:51:44 pmYes, but as long of course that there is some battery charge left. Which is why the "evildoers" know full well that "turning off" a mobile phone is not enough to become untrackable and that they have to remove the battery.That's of course not possible using most modern phones with no removable battery. In which case, they don't use phones or only those dumb phones with no internet and removable batteries.It's interesting to know that many mobile devices like smartphones and laptops do not fully turn off when you seemingly "turn them off" via the OS.For instance, Windows does not turn off computers when you choose to, at least since Windows 11 (may also be the case with Win 10 starting with a given revision). I think there's a way to force Windows to really turn the computer off (at the expense of a longer boot time), but it's not enabled by default.

author: kite31
author_rank: Regular Contributor
timestamp: October 12, 2025, 09:36:43 pm
content: Quote from: SiliconWizard on October 12, 2025, 07:48:07 pmI have a commercial NVR running on Linux. If I tell it to shut down / power off it will stop recording and stop the GUI. However, cameras are still PoE powered and a further step is required to truly power off. Then I pull the plug.This one has no power switch at all, running off a brick. An older one with internal PS behaved much the same, the actual power-off step being the 240V switch.

author: Psi
author_rank: Super Contributor
timestamp: October 13, 2025, 12:54:38 am
content: Quote from: gamalot on October 12, 2025, 04:33:44 pmYou realize there's a new phone coming out every few days and its been that way for a long time.No one has tested them all in this way.It's more probably that some phone somewhere does ping/beacon when off, than it is probably that no phone has ever done this.No one was saying that all phones do this.

author: Halcyon
author_rank: Unknown
timestamp: October 13, 2025, 01:26:52 am
content: Quote from: SiliconWizard on October 12, 2025, 07:48:07 pmThe other consideration, mostly relevant to Apple iPhones is when turning off WiFi and Bluetooth from the "Control Centre", this is only temporary. Under certain conditions, those radios will automatically switch themselves on. To properly disable them, you need to drill down into the device settings. See:https://support.apple.com/en-gb/102412Both WiFi and Bluetooth can still be turned back on independently even is Airplane mode is enabled. It's also possible to identify a device (and the WiFi networks it has previously connected to), by monitoring for probe requests on each WiFi channel.You're right about laptops, Apple devices have utilised sleep mode for quite some time, later versions of Windows have played catch-up. It's mostly about giving the user a "faster" experience. Hibernation on PCs historically has been a bit clunky with mixed results depending on the hardware.

author: gnif
author_rank: Unknown
timestamp: October 13, 2025, 03:13:38 am
content: Quote from: Halcyon on October 13, 2025, 01:26:52 amWorking for a major CPU manufacturer of x86 hardware I can attest that these states are as low power as possible, there is zero interest in trying to keep more alive on the chip then is required (the market is super competitive, it's simply required to stay relevant). When in deep sleep states the regulators for the various IP blocks are turned off completely. Unless it was suspend to disk, the only part major of the CPU that stays alive is the memory controller, at a slow clock, just enough to keep refreshing the RAM to prevent data loss. A tiny bit of the budget goes towards keeping scratch registers valid for state restoration on resume, etc.In short, a Windows laptop, unless the vendor of the laptop itself included some specific proprietary hardware to do this tracking, it simply can't happen.Quote from: SiliconWizard on October 12, 2025, 07:48:07 pmWindows 11 still does fully turn off, the technology is called "FastBoot", it's a limited form of suspend to disk. On the next boot it skips full hardware driver enumeration and initialisation, cold kernel startup (paging, session creation, etc.) and re-initialisation of low-level system services. It doesn't need to do these things again because the state was saved and restored from the data written to disk. Windows can also set a flag the UEFI BIOS picks up on that bypasses some of the POST initialisation also, further decreasing boot times making it appear that the system wasn't "fully off" because you don't see the usual POST process on power on.

author: Psi
author_rank: Super Contributor
timestamp: October 13, 2025, 03:44:14 am
content: Quote from: gnif on October 13, 2025, 03:13:38 amah yep, I remember H.O.R.M from back in Windows XP Embedded (Hibernate Once Resume Many). I assume that technology is what become FastBoot.It definitely made the thing super snappy to boot. Which was useful on an embedded device.

author: amyk
author_rank: Super Contributor
timestamp: October 13, 2025, 04:03:10 am
content: I was thinking they were using some huge scaled-up variant of thehttps://en.wikipedia.org/wiki/Nonlinear_junction_detector, given that they have previous experience with passive radiators:https://en.wikipedia.org/wiki/The_Great_Seal_bugQuote from: Psi on October 13, 2025, 12:54:38 amIf they are coming out at that rate it's because they are merely copy-pastes of existing reference designs with some minor editing, and thus behave mostly the same way.

author: radiolistener
author_rank: Super Contributor
timestamp: October 13, 2025, 04:27:48 am
content: Quote from: SiliconWizard on October 12, 2025, 07:48:07 pmWindows can use several power states rather than performing a full shutdown.For example, in sleep (S3) mode, the system keeps RAM powered so that it can resume instantly without reloading memory from disk. In hibernate (S4) mode, RAM is fully powered down, but the entire memory image is stored to disk and restored at the next boot, which takes longer.Regardless of these modes, some subsystems — such as the network interface controller — may stay partially powered to support features like Wake-on-LAN, which allows the computer to power on when a special “magic packet” is received over the network.Additionally, most modern chipsets include a management controller (Intel ME, AMD PSP, or similar embedded microcontroller) that runs its own firmware and remains active even when the main CPU is in standby or soft-off (S5). This controller handles low-level tasks such as power sequencing, remote management, and security functions. It have low-level access to system buses, memory controllers, storage and network interfaces. Because they sit below the main OS and mediate power, DMA and I/O, a management firmware can:- read or modify RAM and persistent storage out of band;- inject code or data that the main OS will execute or map;- present network endpoints or remote consoles invisible to the guest OS;- survive reboots and sometimes remain active when the main CPU is in low-power states.So, the management firmware can enable stealthy, persistent remote access that is difficult for the OS to detect.

author: gnif
author_rank: Unknown
timestamp: October 13, 2025, 04:33:21 am
content: Quote from: radiolistener on October 13, 2025, 04:27:48 amFastBoot is S5, only the critical details in RAM are saved to disk needed to act like a full clean boot are saved, which is why it's so fast compared to S4.Quote from: radiolistener on October 13, 2025, 04:27:48 amIncorrect, S5 is full system off, you can completely unplug the system and still power it back on using the fast boot/resume method. I can attest for AMD at least that the PSP is powered off in S5, as it is in any "soft-off" state. I would expect this to be the same for Intel.

author: radiolistener
author_rank: Super Contributor
timestamp: October 13, 2025, 04:52:49 am
content: Quote from: gnif on October 13, 2025, 04:33:21 amI think this is not actually correct. S5 indeed means the main system power rails are shut down: the CPU, DRAM, GPU, and most peripherals are unpowered, and the OS has fully terminated. However, standby rails such as +5 VSB or +3.3 VSB remain active. These supply low-power subsystems like the Embedded Controller (EC), power-management logic, and parts of the PCH.The AMD PSP (and similarly, the Intel ME) typically enters a deep sleep state in S5 and usually performs no active tasks. That said, it’s not always fully depowered — whether it retains minimal standby power depends on board design and enabled wake sources (like Wake-on-LAN, RTC-wake, PME signals, etc).The fact that a system can still power on from S5 through a network event or timer implies that at least a portion of the platform controller logic remains powered and capable of reacting to external triggers.

author: Sorama
author_rank: Frequent Contributor
timestamp: October 13, 2025, 05:23:52 am
content: Maybe we should stay on topic, which is about mobile phones and not Windows.

author: Bud
author_rank: Super Contributor
timestamp: October 13, 2025, 06:26:04 am
content: The lost family's phone signal that started this thread was registered on Sep 28. So no mistery here, it was not after 12 days.