Proximity detectors were devices which glowed and pulsated when near others of the same devices. They could be configured to only glow in the presence of a certain group of other devices.
Proximity detectors were developed by the government of the Turkana IV colony in the early 24th century and implanted into the chests of members of the rival factions Coalition and the Turkana IV Alliance. They were meant to deter trespassing on enemy-controlled territory and therefore violence. Furthermore, these 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.
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")