Radar detectors, TV tuners, and other such gizmos use a "phase locked loop" to latch onto and follow the frequency of the signal. This requires the tuner to have an internal oscillator running at the same frequency as what it's trying to tune. Those oscillators give out trace RF emissions. That's what radar detector detectors pick up on. Likewise, it's what the roaming TV detector vans in the U.K. use to notice that you have a TV set where you didn't pay your TV tax.