According to the American Defense department some mysterious coins were found in contractors that were traveling trough Canada (three separate events were registered early in 2006). Why the coins were mysterious you might ask? Because apparently they contained small radio frequency transmitters inside them, with the purpose of spying.
Yahoo news has an interesting article about the deal: “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. The metal in the coins also could interfere with any signals emitted”.
The article also commented that “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.” (no shit man!).