The solution built in R&D (as far as the rumour goes) was much simpler. There were only a few channels (typically 4) and this would have been all analog broadcast. The much simpler signal that TVs "leak" is light.

So look at the pattern of changing colours through curtains, and compare to a TV in the van with something to diffuse the light over the top.

Of course, the even simpler approach is to tell people you can do this and just send around / threaten to send around random vans.