TV Detector Vans

Since television licenses first appeared in the UK, the TV Detector Man has been a ghoulish spectre who can allegedly track down telly-watchers by means of a revolving thingamy-wotsit on top of his detection van. The vans had a mysterious, faintly disturbing Orwellian aura about them.

The majority of vans were no doubt just dummies, designed purely to be highly visible and scare people into buying licenses, but technically the "real" vans could in fact detect TV use.

A television behaves like a low-powered transmitter, emitting several types of electromagnetic wave. With a highly directional aerial - the sort you get on the top of TV detector vans - these signature waves can be picked up nicely.

The technical limitations are such that the vans can't just trawl through the streets - they have to stop outside the house in question and point the aerial at it. 

This isn't really a problem, because the real vans were likely to only get sent out to gather evidence that a specific unlicensed household was using a television.