It doesn't even take a particularly high voltage, just a reasonably high frequency. It is, for example, technically possible to monitor traffic over ethernet via radiated emissions at quite a distance. Granted, it takes very sensitive equipment, but it's possible. TV or video is simple by comparison, especially since they're never actively shielded anyway.

That said, 99% of the TV detector vans are dummies, I seem to recall there's only perhaps 2 real ones in existence. They just send out demands to everyone without a license on the basis that you must have a TV. In addition, when you purchase a new video recorder, TV set, or DVD player, the shop is required by law to note your name and address which is passed along to the TV licensing people. The license covers up to 3 TVs per household, and portable receivers are exempt if they are entirely battery powered and have a screen size below (I think) 4 inches.

Note that while shops are required to take a name and address, they never check it and don't care if it's not YOUR name and address

pca