The tiny chips that are implanted in pets and the like respond to a local detector to transmit data. A "wand," or something like that. They are not detectable at any useful distance.

Actual transmitters and antennas that can be detected at a distance are still pretty big to be implantable. At least the size of a pacemaker battery cartridge, and with similar power needs.

It would take terrorists a very short time to adapt to this, either by shielding rooms from radio, or in extreme cases, removing the transmitter surgically.

Obviously, any transmitter that is simply worn or carried would be quickly destroyed.