This is shaking my world view — although I find it more plausible that (as responses to https://www.theguardian.com/notesandqueries/query/0,5753,-22440,00.html claim) they _did_ work until about 10-20 years ago, by detecting RF emissions from the local oscillator inside the TV. Ross Anderson, at https://www.cl.cam.ac.uk/~rja14/Papers/SE-15.pdf , notes:
During [..] World War II, radio engineering saw advances in radar, passive direction finding, and low-probability-of-intercept techniques, which I’ll discuss in the next chapter. By the 1960s, the stray RF leaking from the local oscillator signals in domestic television sets was being targeted by direction-finding equipment in “TV detector vans,” in Britain, where TV owners must pay an annual license fee that is supposed to support public broadcast services. Its use has since expanded to satellite and cable TV operators, who use detector vans to find pirate decoders. Some people in the computer security community were also aware that information could leak from cross-coupling and stray RF (see, for example, [259, 791]).
Sandler wants to be able to explore the code running her device for programming flaws and vulnerability to hacking, but she can’t. “Because I don’t have access to the source code, I have no power to do anything about it,” she says. In her eyes, it’s a particularly obvious example of a problem that now cuts across much of modern life: proprietary software has become crucial to daily survival, and yet is often locked away from public exploration and discussion by copyright.