By now you may have seen the story about radio transmitters found in Canadian coins:
Each is virtually identical to the others, but the meat of the articles is:
Coins containing tiny transmitters have mysteriously turned up in the pockets of at least three American contractors who visited Canada, says a branch of the U.S. Defence Department.
There's an implication in the article that the U.S. Defense Security Service's report "Technology Collection Trends in the U.S. Defense Industry" is the source of this information, but it's not clear because the journalist didn't explain that bit very well.
That's not the only thing the journalist goofed up on: there's speculation in the articles (apparently from the journalist) that the idea is foolish because someone can merely drop the coin in a vending machine. This is not true. All you have to do is imperceptibly alter the weight of the coin and the Coke machine spits it back at you. You shrug and reach for another quarter.
Though the transmitter type is not named, the article further speculates that it might be an RFID tag. This seemed ridiculous due to the short range of transmission but according to a Wikipedia article on RFID:
Active [RFID] tags typically have much longer range [than passive tags] (approximately 300 feet) [and] the smallest active tags are about the size of a coin and sell for a few dollars.
Still, regardless of what's transmitting and how, nothing gets around the basic problem that these are coins. You're not going to have them for long so it seems like it should be easy to track their source: where did the contractors buy stuff in the past few days and receive change? Of course, if a pickpocket were to plant the coins (who's going to be surprised by a coin in their pocket?), that might fail. This also raises the question of how long a given coin tends to remain in an individual's possession, but I've no idea.
Whatever the transmitters are, they're short-ranged and a controlled environment seems best for them, but if you have the contractors in a controlled environment, why use coins at all? The implication is that it's presumed one can track the coins in an uncontrolled environment, but there's a lot of hand-waving there.
I doubt we'll hear more about this, but I'm quite intrigued.