TV detector vans are a myth
(After many years of public broadcasting being charged if you have a TV -- or even a PC or mobile phone with TV capability -- the charges now go through the normal tax system, so you no longer have to worry about the real TV inspection people asking to come in to see if you have a TV!).
Superheterodyne receivers including the ones in TVs have a local oscillator that's mixed with the received signal. This oscillator leaks out a bit and can be detected at short ranges.
These days there's much more interference and TV is not distributed over the air anymore usually. So it is no longer a thing.
The thing is though, it's usually not needed. All they have to do is go to the houses without a license and either look through the window or listen for the sounds of a TV show.
I wish they'd just abandon the concept of a TV License though. Everyone has a media capable device these days and the administrative load of all this license crap is not worth the savings for the 5 people that really don't have a TV. In other words, just take it out of the taxes.
In the Netherlands they have done this for decades but when I lived in Ireland there was still a really expensive license. For which you get 2 really mediocre channels (IMHO)
The funny thing was the ads on TV. They focused not so much on the huge fine of more than a thousand euros but on the shame of being seen in court without a license. As if anyone gives a F... Lol. Piracy was really rampant when I was there and people even networked their wifis together over long distances to share movies.
You could just about make a speculative case for IF and flyback transmission in the old analogue days, although triangulation would have been a serious problem.
Today's digital systems are much quieter.
But the best argument is that even if they worked it wouldn't make a difference. A large team of inspectors has a list of properties without a license and they're checked in person. A complex detector van does nothing to make this easier - as proven by the complete lack of detector evidence in court cases.
“ Modern efforts to detect licence evasion are shrouded in mystery. Modern flatscreen displays receiving digital television signals do not emit as much radio frequency interference as older designs, and any such signals detected are less easily correlated with broadcast television. An LCD television in the home can just as easily be displaying output from a video game console or an online streaming service, with both being usage cases that do not require the owner to pay a licence fee. Based on an alleged BBC submission for a search warrant in recent years, there may be optical methods used in which reflected light from a television in a viewer’s home is compared to a live broadcast signal. The BBC declined to answer the Freedom of Information request with any details of their methods, other than to say they have employed vehicles and handheld devices in enforcement efforts.”
https://hackaday.com/2021/01/18/tv-detector-vans-once-prowle...
The inspectors can't do much if you refuse to identify or incriminate yourself.
They almost certainly did not work as intended, particularly if you had more than one TV in a 100-metre radius of the van.