Radar detectors are essentially superheterodyne radio receivers. Like any radio, they’re passive devices that listen for microwave signals. The main difference is that they are specially-tuned to detect microwave frequencies transmitted by police radar and laser guns.
If a police car is not equipped with a radar or laser gun, a radar detector cannot possibly locate it. Also, if no signal is transmitted when you pass the police car, there won’t be any signals for your detector to receive.
Police may also use a sinister technique called “Instant-On” or “RF Hold.” During this type of encounter, an officer will wait until a targeted vehicle is within range to send a quick burst of radar to clock its speed. K40 radar detectors are sensitive enough to detect these brief, weak signals, but you have to pay attention and react quickly.