I believe that a German Spy in the U.S. in WW2 was caught when a Radio Amateur working for either a Utility Company or the FCC recognized the buzzing of a faulty Electric Meter to be Morse Code. Radio Amateurs in the U.S. had been prohibited from being on the air.

Following up on Charles W2SH’s comment about messages to the French Resistance from the BBC:

“This is London calling. Some messages for our friends.
The Dice are on the Carpet.
The Dice are on the Carpet.
It is Hot in the Suez.
It is Hot in the Suez.”
Evening, London, 6/5/44
Nous nous Souvenons.
73.
Griff NE3I

Robert Alan Griffiths