That statement seemed a bit odd. What was state of the art in hidden microphones at that time? How did they record the microphones’ signals for later playback?
Not sure what they looked like, but I know that Allen Funt of ‘Candid Camera’ fame was a WWII expert with hidden microphones and wire recorders.
The Mirror is clearer on what transpired:
http://www.mirror.co.uk/news/uk-news/secret-nazi-tapes-shocking-germany-1336922
...British intelligence hid microphones among POWs at Trent Park detention centre in North London, captured every boast, offhand remark and sick joke about the killing of children and new mothers, women being raped and the mass extermination of Jews...