passive bug

A listening device with no power source of its own, which works by means of a strong radio signal aimed at it from outside.

Crypto Museum gives as an example the device known as The Thing.¹

According to the CIA, in 1945 the Soviets presented W. Averell Harriman, the US ambassador to the USSR, with a wooden carving of the Great Seal of the United States – ostensibly as a gift to a wartime ally. Inside was a listening device: a passive transmitter with no battery, power supply or wiring. A high-frequency signal from a car parked outside “illuminated” the device, which then relayed sounds from the room to a receiver.²

The seal hung in the study of the ambassador's residence in Moscow until 1952, when, during George Kennan's time as ambassador, it was discovered in a technical surveillance sweep (see TSCM). The Truman and Eisenhower administrations did not publicise the discovery; only on 26 May 1960, after the U-2 shoot-down, did Ambassador Henry Cabot Lodge Jr. show a replica of the seal to the UN Security Council and explain how the device worked. A Soviet motion to condemn the United States over the U-2 flight was defeated by 7 votes to 2.²