Quote from a BBC press release:
"The new vans are so powerful they can tell if a TV is in use in as little as 20 seconds. And once the television is detected, the equipment - which works from up to 60 metres away - can pinpoint the actual room that the television set is in."
The same press release also explains why you might not see so many detector vans these days. (The vans often operate covertly - there's no external equipment nowadays. This means that they are indistinguishable from any other white van).
The full press release is here:
http://www.bbc.co.uk/pressoffice/pressreleases/stories/2 003/06_june/24/licensing_detector_vans.shtml
Chris