Decades ago the Soviets gifted the U.S. a large wooden plaque which they hung in their embassy. Later they found an electronic listening device inside it they called The Thing. Requiring no batteries, at first agents were perplexed at how such a device could work, but then they realized it's being powered from an external source located down the street. Léon Theremin of musical instrument fame invented the bug.
I have the sneaky suspicion that RFID tags for pets might work in the same manner, allowing electronic eavesdropping from afar. I'm not set up to test such a hypothesis, but it would be interesting if it turned out like the plotline of "The Spy with a Cold Nose" (1966) where you're not "wearing a wire", the wire is surgically implanted. That same year the CIA fielded a remote-controlled cat against the Soviets which had an electronic listening device installed in its ear.
On a side note: A child asks a doctor to install a camera in his head. The doctor prescribes powerful drugs to eliminate such creative thinking. A child asks a CIA agent to install a camera in his head. The CIA agent installs the camera and a bunch of other stuff and sends the kid around the world to catch bad guys.