Covert surveillance WW2 style

The Nazis prisoners bugged by Germans

Carefully listening in on their conversations were fellow Germans of Jewish origin who had fled from the Nazis.
The bugged prisoners were kept in three locations - Latimer House near Amersham, Wilton Park near Beaconsfield, both in Buckinghamshire, and Trent Park near Cockfosters in north London. The first two held captured U-Boat submarine crews and Luftwaffe pilots, who were bugged for a week or two before being moved on to conventional captivity.
The generals, whose numbers eventually reached a peak of 59 as the war progressed, resided in Trent Park until the war ended.
Hidden nearby in each of the three stately-homes-turned-prisons were the pro-British Germans, listening in a place known as the "M room" - the "m" stood for microphoned - where "secret listeners" were glued to the bugging devices.
Historian Helen Fry, who has written a book called The M Room: Secret Listeners who bugged the Nazis., says the information gleaned by the eavesdropping of the German generals was vitally important to the war effort - so much so that it was given an unlimited budget by the government.
http://www.bbc.co.uk/news/uk-20698098