I had an English girlfriend ten years ago and I spent a lot of time talking over the phone with her when she was back in England and I was in the States. One afternoon she freaked out and was telling me that the TV coppers were in her neighborhood and she had to go. I didn’t understand what she was talking about. She hadn’t paid her TV tax! I could not figure out how that scheme works. I guess if your TV is on, they can tell with their antenna vans. But how can they tell if someone is receiving transmissions? Transmitting, okay.
The old CRT tubes emitted strong radiation at the "line scan" frequency. In the U.S. NTSC system, with 525 lines per frame and 60 frames a second it was 525 x 60 = 31,500 Hz. For the British PAL is was 625 lines per frame times 25 frames per second for 15,625 Hz.