In a US government warning high on the creepiness scale, the Defense Department cautioned its American contractors over what it described as a new espionage threat: Canadian coins with tiny radio frequency transmitters hidden inside. The government said the mysterious coins were found planted on US 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 US report does not suggest who might be tracking American defense contractors or why. It also does not describe how the Pentagon discovered the ruse, how the transmitters might function or even which Canadian currency contained them.