VHF Channel Detecrion
NB
You are right to say that no one will monitor if or what channel you are tuned to. It is however a simple task. In order to provide optimum amplification the incoming signal is mixed with a frequency created by your radios internal oscillator to create a fixed intermediate frequency which is further amplified within your radio. This internal oscillator is controlled by the channel selector. Since few commercial radios have adequate shielding to prevent all emissions of this oscillator frequency anyone with a suitable receiver and a spectrum analyzer can tell exactly what channel you are monitoring.
Back when the BBC had a TV monopoly in England they used this technique to determine who was receiving their programing without paying the TV tax. They actually had vans with directional antennas that would drive around to find the none payers.
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