The government said the mysterious coins were found planted on U.S. contractors with classified security clearances on at least three separate occasions between October 2005 and January 2006 as the contractors traveled through Canada.

Intelligence and technology experts said such transmitters, if they exist, could be used to surreptitiously track the movements of people carrying the spy coins.
The U.S. report doesn’t suggest who might be tracking American defense contractors or why. It also doesn’t describe how the Pentagon discovered the ruse, how the transmitters might function or even which Canadian currency contained them.

Further details were secret, according to the U.S. Defense Security Service, which issued the warning to the Pentagon’s classified contractors. The government insists the incidents happened, and the risk was genuine

“What’s in the report is true,” said Martha Deutscher, a spokeswoman for the security service. “This is indeed a sanitized version, which leaves a lot of questions.”