In any building spies had access to there was a game of cat and mouse as (for example) spies would see people with radio equipment scouring rooms through the window as they heard their bugs go pop one by one.

Peter Wright (who wrote spycatcher) was the one who first got to look at The Thing; He actually broke it and had to make a new membrane for the microphone. He also invented (within MI5 at least) their technique for detecting superhet receivers (RAFTER), at one point they apparently were parked next to the soviet receiver.