I keep mentioning the TV Dectector Van, which is something that the TV Licensing folk drive about
Despite what the site where I found this pic called it sinister bbc spy vehicles: TV Detector Van 2006, it is the TV Licensing authority who run it (which is written on the side of this van: http://www.tvlicensing.co.uk). The licensing authority have this to say about the vans:
We also have a fleet of detector vans , plus, our enforcement officers have access to hand-held detection devices capable of detecting a TV within 20 seconds. In fact, we catch an average of over 1,000 people watching TV without a licence every day.
More about all this from the TGV Licensing site:
Of course, I think this is more PR than any actual threat, but they do have these things driving around.