What is the fine-grained topic of the following text: The confidential accounts led to a sensational warning from the Defense Security Service, an agency of the Defense Department, that mysterious coins with radio frequency transmitters 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. Only return the topic.
Radio frequency transmitter coins planted on U.S. contractors in Canada