[Nexus 5X Q&A, Help & Troubleshooting] Alcolawl:
Background:

So last night I went to bed and left my phone on my dresser with 30% battery. The phone normally drains a few % overnight. WiFi on, Location on (high accuracy), Mobile Data On, etc. I wake up this morning to find that my phone is completely dead. I thought it was pretty strange so I connected the phone to the charger for an hour or so and charged to about 96%. I disconnected my N5X from the charger and put it on my computer desk and sat down to play The Division for a little while. After about an hour, I noticed that my phone was still very warm and that my battery had sank down to 88%... I've never had this problem before, even on my current ROM and kernel setup, which I've been running for about a week now.

Problem: Insane battery drain. CPU is Awake 100% of the time, even when the screen is off. No Deep Sleep and No Doze.

I did notice, however, that the phone reported WiFi being on for a large portion of the day where I actually had turned the WiFi off. The gym I go to every day has horrible WiFi and I prefer to use LTE. I usually forget to turn WiFi back on after leaving to go back home and shower. So there's at least a 2-3 hour segment there where WiFi was absolutely not enabled, and yet the battery graph shows that it was....

Culprit:

This mother****er right here. com.google.android.gms. Specifically two alarms: ALARM_WAKEUP_LOCATOR and ALARM_WAKEUP_ACTIVITY_DETECTION. I'm not so sure about the latter, but the Alarm_wakeup_locator is actually waking the phone up at least twice a minute, which is insane. It's massacring my battery life as well.

Solution?

I've never had a battery drain problem like this before. In fact, I haven't had a drain problem like this in at least a year or two. I have no idea what to do to fix this. I've Googled this issue and there doesn't really seem to be a straight answer.

I think it has something to do with Google Now or Google Play Services, perhaps a location setting, but I've tried disabling Location for 2 hours now and it has not fixed this issue. I have considered perhaps disabling a service but I can't identify which service exactly is causing the issue.

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Update! - March 22nd, 2016

After a second clean wipe and a switch to Pico Gapps, the problem was resolved. This gave me a barebones operating system in which the CPU was sleeping just fine when the screen was off. No wild wakelocks or alarms to report. One by one I reinstalled my favorite apps, avoiding almost anything by Google -- No drain / keep awake issue reoccurred. My phone is now back to normal with only one thing missing. Google Now. The Google app and Google Now Launcher are both NOT on my device and my phone is achieving some seriously sick battery life and SOT. I am confident that if anyone else is facing these issues that the culprit is Google Now and the Google Search app. Those experiencing this issue should try removing these apps if possible or clean wipe and install with a very minimal Gapps package, just as I did.