During WWII when the Brits captured a German who might have information they would want, they brought them over to England for interrogation. The interrogation was not brutal, and while they were being interrogated the prisoners were housed in rather spacious, roomy, well-furnished mansions with other “high value” prisoners.
Those houses were totally and completely bugged, a new technique at the time. The bugs were constantly monitored, and when two or more prisoners started talking about anything interesting it was recorded on 78 records.
Now, everyone would know they were being watched and heard 24/7, but in those days no one thought about it. That technique was so successful it was continued until the end of the war. And kept secret in case it should be useful again.