A spying bug that passively transmits your voice.

The Thing, also known as the Great Seal bug, was one of the first covert listening devices (or "bugs") to use passive techniques to transmit an audio signal.

It was concealed inside a gift given by the Soviet Union to W. Averell Harriman, the United States Ambassador to the Soviet Union, on August 4, 1945. Because it was passive, needing electromagnetic energy from an outside source to become energized and activate, it is considered a predecessor of Radio-Frequency Identification (RFID) technology.

The Thing was designed by Soviet Russian inventor Léon Theremin, best-known for his invention of the theremin, an electronic musical instrument. (...)

https://en.wikipedia.org/wiki/The_Thing_(listening_device)