listening devices

“SONOBUOYS: acoustic listening devices dropped in the water by ASW or USW aircraft.”

Terror At Dawn

“This was the Holy of Holies, a conference room for very private conversations, the concrete room and communications center laboriously built by trusted SI officers, British only, to ensure against enemy listening devices being inserted into the walls — the whole structure tested weekly by Special Branch experts — in case some were somehow infiltrated.”

Noble House

“For one thing, there were the listening devices in Dr. Beck's home.”

Tell No One