When I was a kid I used to hook a coil of wire and a diode to a piezo earphone. I then listened to the emissions of various devices in my house. My Amiga 500 was particularly interesting.
I ran across a few projects that do the same thing but add an opamp and recording so you can generate sounds for electronic music.
Isn't that effectively all the EMF? You may be missing a prefix.
Edit: huh, turns out these frequencies do have some applications: https://en.wikipedia.org/wiki/Super_low_frequency
In any case, I would want to detect the emissions from the CPU, memory bus and SD card traffic. Not all cameras are WiFi. Sure, that won't detect passive listening devices or other advanced techniques, but most people now are worried about video and that will have certain characteristic emissions.
Edit: Something like CamRadar, presented here: https://dl.acm.org/doi/10.1145/3569505