Comment by srdjanr

You could only realistically detect this in a lab.

You need to actively listen at multiple frequencies at the same time (gsm/lte/5g have many different bands, + wifi/bt at 2.4/5/6 ghz and their bands). And you need to remove interference from all other devices to conclude that signal is coming from ledger (so any home isn't an option).

On top of that, you need to be listening at the exact time it exfiltrates data (it might intentionally wait for some time after using device), and you need to have the compromised device (which the hardest part here - you could test 100s of devices and not find anything).

So I think this could only be caught with a large scale, recurring lab testing, far from "trivial to detect".