I read years ago that the British made a breakthrough in the "battle of the beams" when a crew member said to another "they wont find it" as they were taken away from a German bomber forced down, one of the police spoke German and the aircraft was taken down to the rivets to find "it".
It was overheard via a "bugged" room when 2 members of an aircraft from KGr100 (elite Luftwaffe night pathfinder group) were shot down and captured. The "it" was the X-Gerat bombing device (the follow-on to Knickbein.) It was built into a regular Lorentz electronic landing aid (common in most bombers) which was modified to be especially sensitive but otherwise look like a regular unit. R.V. Jones got wind of this and asked the post crash RAF investigator if there was anything unusual about the electronics in the downed aricraft and his reply was just that the Lorentz unit was "way more sensitive than any he had seen." This is why it was not obvious and one of the two captured Luftwaffe officers made that statement to the other. Jones' book (Most Secret War) has this episode in it. On several occasions, good intelligence was obtained by placing captured Germans in a room by themselves and then secretly "listening in" to their talk.