[Kamog]

They can detect you receiving TV transmissions because a TV receiver contains a local oscillator that is used to downconvert the television signal from high frequency to low frequency. You know, when they broadcast the signal, they use a carrier that is much higher frequency than the original video signal, and when you receive the signal, you have to convert the frequency down again to be able to watch it. The oscillator in the TV receiver leaks out a signal that can be picked up by antennas on those vans driving by.