A proximity detector used by the Coalition on Turkana IV
The two rivaling factions on Turkana IV colony, the Coalition and the 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.
An activated Coalition proximity detector.
The Coalition used yellow implants, the Alliance's implants were green. The devices were implanted directly underneath the skin, usually on the chest.
Similar devices were also used on Manu III. (TNG: "Legacy")
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")