They don't use non-descript vans. They want you to know who they are. Now, whether the vans can actually detect anything is a different matter. Some believe they are just a visual deterrent, and don't actually do anything beyond looking scary.
Cynically it gets your auntie talking, brings TV license to front of mind.
It's doubtful they ever actually did such a thing.
There is some kind of 'detector' mentioned online that can apparently look at a window and see the light of a TV flickering on the glass! Judges buy this bs and issue warrants to search.