RF signal detectors can pick up what frequency the strongest signal in a range is broadcasting at. I had a cheapy ebay one that at a cost of $30 could pickup my

RF signal detectors can pick up what frequency the strongest signal in a range is broadcasting at. I had a cheapy ebay one that at a cost of $30 could pickup my phone ringing or on a call/text on its LTE frequency from about 6 feet away, and that was with it using an untuned antenna as the detector was able to pick up between 100mhz-2500mhz. I'd imagine with a much higher quality detector, a tuned, directional antenna, and a narrower range you needed to cover that you could up that distance several times from the 6 feet I got. We also had the police employing radar detector-detecting devices 15 years ago that could detect every single radar detector on the market in 2004, and none of those radar detectors were intentionally transmitting a signal: https://en.wikipedia.org/wiki/Radar_detector_detector https://en.wikipedia.org/wiki/Radar_detector_detector