How about this: http://en.wikipedia.org/wiki/Thing_(listening_device) http://en.wikipedia.org/wiki/Thing_(listening_device) Powered by an external radio beam.

How about this: http://en.wikipedia.org/wiki/Thing_(listening_device) http://en.wikipedia.org/wiki/Thing_(listening_device) Powered by an external radio beam. Transmits by modifying that same beam and reflecting it back. No internal power source. No active transmission. Weighs 1.1 ounce and was built in 1945