Subspace proximity detonator

From Memory Alpha, the free Star Trek reference

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 and 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 conflicts and were responsible for more than 200 Starfleet casualties. (TNG: "Suddenly Human")