They once had at least one working detector van. It's in a museum somewhere now. But the suspicion is that they also had a number of others painted up with nothing inside. No figures have ever been published showing the number of prosecutions as a result of detector van evidence, and there have been no known cases.
Today, they state that their detector vans are stealthy, so don't have any writing on them. The idea being that any plain van, or possibly some with business names on them, could all be detector vans. Everyone not paying them should fear every van that drives past.
This sort of nonsense is just ridiculous in any sane democracy.