> Apparently they work by sending out another "channel" that all TVs will recieve, no matter what channel they're tuned in to. The TVs then amplify this signal, and that's what the vans detect. That's also why monitors aren't detected, as they don't resonate at that frequency.
>
> Link: The alien sensor device thing, and the theory thereof

Ok, I guess that means the detector can detect your telly regardless of whether it's on or off. Well, that particular link explains the theory, but doesn't explain the technique. How do they know it's *your* TV at that location, and not your neighbour's immediately behind the adjacent wall. By triangulation, like shadowfax suggested? Then they'd need exact up-to-date floor plans of every building they're surveying, and thermal imaging units to detect the wall divisions, and... Grrr. If there's a lot of old iron girders or metal brackets in the wall brickwork, the TV's resonance frequency probably will be messed up and undetectable.

Maybe this is making it too complicated. Perhaps the detection team just assumes that if they see one or two tellies somewhere on the premises, SOMEONE's got to pay the licenses and it doesn't matter who.

Wolf "How about that scene in Total Recall, where Arnold outfoxes the detectors' tracer bug up his nose by wallpapering *his head* in aluminum foil? Augh!" spirit