Major Mad Max, almost all radio receiving devices have a local oscillator to modulate the incoming signal (but I've forgotten why this is necessary for the moment). This circuit produces rf energy - some of which can be emitted. The principle was used by MI5 to catch Soviet agents listening to coded broadcasts - they would drive around the area in question with the equipment in a vehicle around the time Moskva Centre was known to transmit, monitoring the Soviet signal themselves as a guide. When they detected a local oscillator signal simultaneous with the Soviet broadcast, they knew someone nearby was listening.
They further developed this to include active detection, in which they would ping the target with a burst of noise on the frequency in question to see if they got a signal - used to confirm the detection at a specific spot. The project was called RAFTER and was large - they even flew airborne-RAFTER missions over London to get a general idea of where a spy was before placing the monitors on the ground to pinpoint them.
It was a Peter Wright thing, and it's all in Spycatcher anyway...I suspect strongly that Stopstart's radar detector, a receiver, was counterdetected by the same method. Also, I'm of the view that TV detector vans work on this principle.