They might be in an otherwise radio silent part of the world though. And how hard is it recording the electrical interference of a device from the same room and then from other rooms, to establish the variability of its electrical "interference/signal" properties?

Knowing about the Theremin, a Russian electrical instrument invented in 1919, is similar to walking around a house with mains electricity is an audible example of the interference in our homes which we interact with. https://www.youtube.com/watch?v=-QgTF8p-284

Could even do human detecting wifi signals. https://discovery.ucl.ac.uk/id/eprint/10103371/1/Passive%20W...

If someone can detect multiple wifi signals, the are effectively inside a phased array radar system.