We always found some.
The funniest of these was found in a hotel we stayed in during a 2 week vacation. It was in Yalta. There was a chandelier in the room with a little bowl shapped object at the bottom. At the very bottom of that bowl was a little ball. My brother and I noticed something funny about the ball and pulled on it. The glue was apparently not quite set yet and two little wires led into the ball. My brother and I (sans any knowledge or consent) decided to try to play a joke on whomever was listening. We went outside and got two bricks, headed back to the hotel room and then whispered into the ball the Russian equivalent of "Hey, James Bond!" and then smashed the microphone. We then went out to dinner with our parents and when we returned, sure enough, a new "chandelier ball" was glued in place.
My brother and I always hoped that whomever was listening turned up the volume and leaned forward in his seat before hearing the bricks clack together.
Most of the time, IME, the mic's were easy to spot. They were usually big - about the size of a Kennedy half dollar. And, too, like your friend, the agents assigned to us (with one hilarious exception) were friendly and reasonably open about what they were doing. In fact, that vacation trip to Yalta? Our agent from Leningrad was curiously at the same beach we visited for the entire two weeks, often spreading his towel right next to ours.
