In Spycatcher Peter Wright mentions that microwaves can be used to detect sound-waves in a suitable tuned object without the need for it to have an active power source, and that such a device had been successfully tested as early as the 1950s.
It arose out of the work I had been doing for Brundrett’s committee on resonance. I had spent a lot of time researching ways in which innocuous objects, like ashtrays or ornaments, could be modified to respond to sound waves when radiated with microwaves of a certain frequency. If a system could be perfected, it promised enormous advantages. The object itself would carry no transmitter or receiver, so detection would be virtually impossible. By 1956 we had successfully developed prototypes, and decided to attempt an operation against the Russian Embassy in London.
One of MI5’s agents at that time was the MP Henry Kirby, who had frequent dealings with the Russian diplomatic community. The plan was simple. MI5 would design an ornament modified to reflect sound, and Kirby would give it as a gift to the Russian Ambassador.
It seems like this specific plan petered out (no pun intended), but given that the prototypes were successful in capturing sounds, were they ever deployed against a foreign power?