The two rival factions on Turkana IV colony, the Coalition and the Turkana IV Alliance, used proximity detectors implanted in their bodies, which glowed and pulsated when faction members trespassed on enemy-controlled territory or came in contact with opposing forces.
The implants were equipped with a micro-explosive inside that would explode on contact with air in order to prevent the colonist of the planet from removing the implants.
The Coalition used yellow implants, the Alliance's implants were green. The devices were implanted directly underneath the skin, usually on the chest.
A different kind of proximity detector was also used by Starfleet. Lieutenant Commander Susanna Leijten suggested installing proximity detectors and infrared sensors on Tarchannen III to locate her former colleagues of the USS Victory. (TNG: "Identity Crisis")