Y'know, it's not *totally* beyond the realm of possibility.
The Soviets did something roughly similar once, during the Cold War (late '50s, I think this was), that was well-documented. The bug in that case was a hanging plaque of some kind that was presented to an American embassy staffer and hung on the wall in his office. No batteries, no wiring, just (IIRC) a pair of tubes that made the aural part of a resonant RF circuit. If it was 'illuminated' by a radio beam at the right frequency, it would re-radiate a fractional-power reflection of that signal with whatever the voices of anyone speaking in that office modulated on top of it. Took forever to find the darn thing, b/c the remote operators could hear the bug-sweeping crews running and kill the illumination beam. Even after it was found, they couldn't make heads or tails out of it until the diagram was shown to one of MI-5's technical whiz kids.
So, pulling the same trick with a coin-sized object might not be totally out in tinfoil-hat country. We already have RFID tags working at very small sizes and w/o any internal power, so there's no reason we couldn't layer a microphone and some superhet circuitry atop an existing RFID base. Of course, it might not be worth the effort -- clarity would be poor, and countersurveillance techniques take that kind of thing into account these days. But probably the biggest hazard would be the humble vending machine: after all, how do you keep the target of your bug from getting thirst and dropping the coin into the nearest Coke machine?
The Soviets did something roughly similar once, during the Cold War (late '50s, I think this was), that was well-documented. The bug in that case was a hanging plaque of some kind that was presented to an American embassy staffer and hung on the wall in his office. No batteries, no wiring, just (IIRC) a pair of tubes that made the aural part of a resonant RF circuit. If it was 'illuminated' by a radio beam at the right frequency, it would re-radiate a fractional-power reflection of that signal with whatever the voices of anyone speaking in that office modulated on top of it. Took forever to find the darn thing, b/c the remote operators could hear the bug-sweeping crews running and kill the illumination beam. Even after it was found, they couldn't make heads or tails out of it until the diagram was shown to one of MI-5's technical whiz kids.
So, pulling the same trick with a coin-sized object might not be totally out in tinfoil-hat country. We already have RFID tags working at very small sizes and w/o any internal power, so there's no reason we couldn't layer a microphone and some superhet circuitry atop an existing RFID base. Of course, it might not be worth the effort -- clarity would be poor, and countersurveillance techniques take that kind of thing into account these days. But probably the biggest hazard would be the humble vending machine: after all, how do you keep the target of your bug from getting thirst and dropping the coin into the nearest Coke machine?