Subspace proximity detonators were a piece of military technology employed by several species, including Koinonians and Talarians. Starfleet tricorders were unable to detect the detonators, as were ship's sensors.
Subspace proximity detonators were put into small bomb-like devices used by the Koinonians during the Koinonian Wars. After many years, the devices were still in working condition. Furthermore, normal tricorders could not detect them. The USS Enterprise-D's ship's archaelogist Marla Aster was killed by such a device when exploring caves on the Koinonian homeworld. (TNG: "The Bonding")
The detonators were part of self-destruct devices sometimes installed aboard Talarian observation craft during the Galen border conflict and were responsible for 219 casualties over a three day period. (TNG: "Suddenly Human")