How do intelligence sweep engineers know whether your phone is bugged?

A phone bugging detection service (or, as it’s known in the trade, “intelligence sweeping”) is one of the most secretive businesses in the world, topped, perhaps, only by the phone bugging business itself.

Phone bugging detection equipment is usually changed by the service every three months, and often sooner, but then, so are the phone bugs.

Intelligence sweep engineers employ a wide variety of equipment. Although no functional countermeasures can be used to prevent the original placement of a bug, which consists of a small radio transmitter, the intelligence sweeper utilizes microwaves to determine whether or not a phone is bugged and, if it is, where in the immediate vicinity the bug resides.

The bugs themselves operate on a particular radio frequency which allows the conversation to be broadcast to another party; the intelligence sweeper’s microwaves are used to try to spot the exact location of the radio waves.

Using a hit or miss system of triangulation (determining a position by taking bearings to two fixed points of a known distance apart and computing the dimensions of the resultant triangle), the sweeper eventually, arduously, pinpoints the location, and then simply performs a physical search to discover the transmitter.

An alternative to contracting for a private, secretive intelligence sweep is to call up the phone company. American Telephone and Telegraph has provided, free of charge, a phone bugging detection service for all its customers. Ask the company to check your phone and it will.

10,000 subscribers per year called for this service in the 1980s, and AT&T in turn detected about 200 listening devices.