Термен опять попался в Вики - на этот раз в статье про RFID:

В контексте резонансного радиомикрофона:

https://en.wikipedia.org/wiki/Radio-frequency_identification#History
In 1945, Léon Theremin invented a listening device for the Soviet Union which retransmitted incident radio waves with the added audio information. Sound waves vibrated a diaphragm which slightly altered the shape of the resonator, which modulated the reflected radio frequency. Even though this device was a covert listening device, rather than an identification tag, it is considered to be a predecessor of RFID because it was passive, being energized and activated by waves from an outside source.

Все-таки забавно - чего он только не напридумывал - а в массовое сознание вошел как фрик-изобретатель странного электроинструмента :)