At the beginning of WWll it was assumed there would be enemy agents operating secret wireless from Britain. Numerous radio amateurs were instructed to listen for them and report back. Although not many were actually detected, some were heard and quickly dealt with.
However, the radio operators heard other signals which were unusual and it was soon realised they were from Nazi operations. Teams of intercept operators both military and civilian were set up to monitor these communications and the results were eventually decoded at Bletchley Park. This intelligence proved to be most useful to the battlefield commanders in bringing the war to a close