In the UK, you need a license to watch TV. The proceeds from the license fee fund the state broadcasting company, the BBC. In order to enforce this, the government sends out TV license inspectors, who go into unlicensed properties to check whether there's a TV present. Of course you don't actually have to let them in; they have no legal power! They are real. I had one visit once. He came in and asked what my TV was. I (truthfully) told him we watched Netflix and Youtube on it, and didn't own a TV box. He left.

They also have TV detector vans, which drive around and detect which houses have television sets.

On The Existence—or Non-Existence—of TV Detector Vans

On the one hand TV detector vans don't exist. Wikipedia calls them an "urban legend". It goes on to say that no detector van has ever enforced a TV license fee. This appears to be true. The BBC has never registered a patent on the operating principles of a TV detector van. The principle of detecting a broadcast receiver is itself pretty fraught. How do you detect whether something is absorbing a signal?

On the other hand, they do exist. There are pictures of them and eye-witness accounts. The Post Office Electrical Engineer's journal details the successive development of multiple types of detector: initially, old-timey cathode ray TVs could be detected by their magnetic field (which deflects the electrons to a specific point on the screen, producing the image). Then later on, receivers could be detected by the signals produced by their local oscillators (sometimes, the easiest way to detect a 50 MHz signal is to produce your own 50.01 MHz signal and detect the 10 kHz interference waves). 

To make it a slam dunk, in 2013, the TV license inspectors applied for a warrant (to actually legally enter a property) based on some evidence from their detector vans. Several Freedom of Information requests have reproduced the exact text of the warrant, and it's this:

5. A television display generates light at specific frequencies. Some of that light escapes through windows usually after being reflected from one or more walls in the room in which the television is situated. The optical detector in the detector van uses a large lens to collect that light and focus it on to an especially sensitive device, which converts fluctuating light signals into electrical signals, which can be electronically analysed. If a receiver is being used to watch broadcast programmes then a positive reading is returned. The device gives a confidence factor in percentage terms, which is determined by the strength of the signal received by the detection equipment and confirms whether or not the source of the signal is a “possible broadcast”

Pretty cut-and-dry here. The warrant was granted, and the TV was found.

I'm sure some of you can already guess the punchline. That long string of technobabble is just describing a camera. They even call it a "camera" later in the warrant application. Whoops. Maybe they don't exist.

So what's the answer here? Either way it's kinda nuts. Maybe the BBC actually did develop and build TV-detecting technology just to build detector vans and drive them around detecting TVs. Maybe they published multiple articles explaining how one might work, built several distinct generations of detector vans which looked sci-fi but also different, and then put their regular TV license enforcers in them, and then lied about it for years up to this day.

I think I lean towards the original generations being real. I think the British Government of the 1950s had the chutzpah to actually build a few of these things. At some point, the technology ceased to be viable (and indeed very few people claim to have seen a TV detector van today) and were quietly wound down. They probably kept up the hoax for the purpose of deterrence. If this is true, I think it's unlikely but not impossible that they ever labelled non-functional vans with "TV Detector" and drove them around. I think we can explain the lack of direct convictions with the fact that a TV inspector van would only ever have been sufficient evidence to get a warrant, not an actual conviction (in the 2013 case they had pictures of the guy and still had to get a warrant).