In 1945 the Soviet Union gifted a wall hung artwork to the USA embassy which passively transmitted audio signal from the room with no power supply or active electronics.
No internal power supply. Very important distinction.
"The device, a passive cavity resonator, became active only when a radio signal of the correct frequency was sent to the device from an external transmitter."
But the Thing was a transmitter. A receiver could be entirely passive and very small. It could easily fit in a dental implant. (There have been reports of people hearing AM radio broadcasts through their fillings.) But the risk of detection would be very high.
https://en.wikipedia.org/wiki/The_Thing_(listening_device)
in the last 77 years, I imagine this type and similar technology will be much more miniaturised and effective.