A proximity detector was a type of sensor device in general use in the 24th century.

In 2367, Lieutenant Commander Susanna Leijten suggested installing proximity detectors on the surface of Tarchannen III as an aide in locating several former colleagues from the USS Victory. (TNG episode: "Identity Crisis")

As well as having military applications, proximity detectors were also incorporated into anbo-jyutsu rings and would detect a players movements. When a player is approaching the edge of a ring the detector would send out an alarm to alert them. (VOY short story: "Winds of Change")