It was certainly possible to detect the operation of TVs once; the old-school CRT boxes had big oscillators in that emitted EM radiation. I don't know about mod

It was certainly possible to detect the operation of TVs once; the old-school CRT boxes had big oscillators in that emitted EM radiation. I don't know about modern TVs; for all I know they have a different method of operation now. That said, it requires not just the equipment to do so, but also the competence and training, and whilst it's not impossible, licence inspectors are just whoever Capita managed to hire that month; maintaining a cadre of competent people to operate that equipment would be a headache and an expense that Capita simply aren't interested in. Far, far cheaper to simply have that big list of addresses (I understand they've got a list of every address in the UK that ever had a TV licence associated with it) and just assume that every address on that list that doesn't currently have a TV licence is a licence-dodger. Every so often it presumably gets pruned for addresses that no longer exist, but otherwise the rate of false positive will be a minor annoyance in the scheme and certainly not worth the hassle and expense of more sophisticated methods. Threatening letters and intimidating adverts do the job.