TV detector vans

From the Guardian

Interesting. The common answer seems to be that they detect the frequency pitch that your television is emitting, so they can tell if you are watching the tele and even what channel. Tune out if you are sick of paying the tv tax for sub standard programming.

Reading more, many think it is address based (quite likely) or they don't exist (which is address based, no license on the database they pay you a visit, though we haven't had anyone thus far). The mystery continues...