Harris said likely candidates include foreign spies who targeted Americans abroad or businesses engaged in corporate espionage. "There are certainly a lot of mysterious aspects to this," Harris said.

Experts said such tiny transmitters would almost certainly have limited range to communicate with sensors no more than a few feet away, such as ones hidden inside a doorway.

"I'm not aware of any (transmitter) that would fit inside a coin and broadcast for kilometers," said Katherine Albrecht, an activist who believes such technology carries serious privacy risks. "Whoever did this obviously has access to some pretty advanced technology."

Experts said hiding tracking technology inside coins is fraught with risks because the spy's target might inadvertently give away the coin or spend it buying coffee or a newspaper.

They agreed, however, that a coin with a hidden tracking device might not arose suspicion if it were discovered loose in a pocket or briefcase.

"It wouldn't seem to be the best place to put something like that; you'd want to put it in something that wouldn't be left behind or spent," said Jeff Richelson, a researcher and author of books about the CIA and its gadgets. "It doesn't seem to make a whole lot of sense."

Canada's physically largest coins include its $2 "Toonie," which is more than 1-inch across and thick enough to hide a tiny transmitter. The CIA has acknowledged its own spies have used hollow, US silver-dollar coins to hide messages and film.

The government's 29-page report was filled with other espionage warnings. It described unrelated hacker attacks, eavesdropping with miniature pen recorders and the case of a female foreign spy who seduced her American boyfriend to steal his computer passwords.

In another case, a film processing company called the FBI after it developed pictures for a contractor that contained classified images of US satellites and their blueprints. The photo was taken from an adjoining office window.

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