TV detection vans are real but they aren't high tech. Some people think they have fancy antennas that can detect your TV. Other people think it's all a bluff and the antennas on the roof of the van are fake just for show.

The truth is much simpler. The antennas on the top of the van are real, but they just receive regular TV broadcasts. The technician watches TV in the back of the van. They also point a parabolic microphone at your window so they can hear what's happening inside the house.

If there's a TV playing in the house, the technician flips through TV channels to see if they can synch up the mic audio with the broadcast audio. If they record a perfect match, that's sufficient to prove that you're watching broadcast TV, not just a recording, VHS, DVD, or streaming service.

Edit: I should clarify, the reason this isn't considered eavesdropping or wiretapping is because the technician isn't actually listening to or recording the audio being gathered from the shotgun mic. They are just observing the results of a comparator that is comparing the TV broadcast waveform with the mic waveform, without recording anything. This allows them to identify identical sound profiles without actually hearing or recording the sound. The machine basically goes "Yep these audio signals match to within an acceptable margin of error"