Listening devices can be passive, getting their energy from radio-waves beamed at them. A clever invention by the soviets that was actually used. It’s also the concept behind RFID.
Most likely, these days, they will bug your phone though.
ah that’s cool. a recording device would still need some kinda power right?
Devices that record to some kind of medium don’t seem like a practical choice, since you’d have to regularly have access to it to get out the sd-card or whatever it records to. And once you’re down that rabbit hole, it might just as well be battery powered.
