TV detector vans
Can pick up the signal from the IF oscillator in a television receiver, and (in the case of analogue broadcasting) can even tell which channel you're tuned to. Digital television makes this a bit harder, but it's still possible if you're watching via Freeview (through the same aerial that is used for analogue) and the big satellite dish on the side of the building makes it pretty obvious if you've only got a Sky subscription.
( K2k6 has a proper job these days, Mon 15 Mar 2010, 13:32, Reply)