The physical principle is that they detect the leakage from the local oscillator in the superheterodyne receiver circuit. I very much doubt that they exist now

The physical principle is that they detect the leakage from the local oscillator in the superheterodyne receiver circuit. I very much doubt that they exist now, if they ever did. It just isn't cost efficient compared to sending out letters and low paid inspectors who peer through windows looking for people watching TV.