Not only are there radar detector detectors, but then there are radar detector detector detectors, and radar detector detector detector detectors. A hilarious arms race.
It gets better than that: there were radar-detector-detector-detectors [1] which were consumer radar detectors that could detect the police radar-detector-detector and shut down.
It's tagged as [citation needed] on wikipedia, but the source [2] does mention this as a feature of some radar detectors. I like to hope it's true.
This cat-and-mouse game has been going on for a long time, even outside of spycraft.
E.g. radar detectors are illegal in several states, so of course there have been devices sold to law enforcement to allow them to detect these illegal radar detectors. https://en.wikipedia.org/wiki/Radar_detector_detector
That's all probably moot now. I think law enforcement has moved mostly to using lasers, which are much more difficult to detect.
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