And if anyone thinks this is a joke, I remember watching these adverts as a child and thinking they really could work out what you were watching on the telly.
This has never been admitted officially:
http://www.tvlicensing.co.uk/faqs/FAQ73
Which is all extremely weird - because it means the BBC spent real money buying a few vans and kitting them out with fake buttons, lights, and switches. And then more real money driving them around the UK and parking them in random locations. To frighten people.
As for Scarfolk - found it a few years ago, thought it was hilarious. Technically it's an example of hauntology, which is borrowed from Derrida[1], but only really makes sense in the grimmer parts of the UK.
https://en.wikipedia.org/wiki/List_of_hauntology_artists
[1] Obviously...
(I may be misremembering but I'm sure those ads were around in the mid-80s too.)
Frankly, I think it's implausible that detector vans were a complete hoax. The BBC undoubtedly had the technological capability to reliably and quickly locate TV sets. I do think it's plausible that the actual use of detector vans was overstated for PR purposes.
It would be one thing to detect an analog TV IF signal, another to triangulate its position precisely enough to identify a specific house in a street or room in a block - given that every other TV in the area would be generating exactly the same IF oscillator signal - and a third to prove the TV was tuned into a program and not being used for pong, a computer, or watching a video.
And it would be unnecessary. The licensing enforcers have always kept a record of addresses without a license. If - like me - you don't have a TV, you get a regular stream of letters threatening prosecution anyway, and an occasional knock on the door.
If you're an enforcer, the easiest way to check if someone has a TV is to look at a house after dark. Most living rooms in the UK are visible from the street.
How did you check?
TVL say they use detector van evidence as grounds for a search warrant, which seems perfectly reasonable. Why argue the technicalities of RF triangulation in court when you can just get a warrant, search the house and collect incontrovertible evidence?