Phase-shifting beacon

A phase-shifting beacon is a device used for sending a signal across distances that cannot be easily detected because of its phase-shifting nature.

In 2402, when Alexander Rozhenko committed suicide to stop Toral from unleashing the metreon wave on Cygnus III, Rozhenko implanted a phase-shifting beacon under his skin. When Toral's men recovered Rozhenko's body from R'kalla Square they were unable to detect the beacon and Starfleet followed the beacon to Toral's hiding place, capturing Toral and his fellow terrorists. (SS TNG (Staying the Course))