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.
The microphone hidden inside was passive and was only activated when the Soviets decided to turn it on. They usually did so from a van parked outside of the ambassador’s house in which they ultra-high frequency beams at the house. Activated, the microphone would transmit any conversations coming from within range of the bug. When they turned the beams off, the bug was virtually undetectable.
Things are much more advanced now, and are everywhere.
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