Proximity transceiver

From Memory Alpha, the free Star Trek reference

A proximity transceiver is a component of a Borg drone's cranial implant. Its function is analogous to a beacon that alerts the Collective of the drone's presence.

In early 2375, Seven of Nine's proximity transceiver activated in response to a 29th century drone, One, created from the merging of her nanoprobes and the Doctor's mobile emitter. One also possessed a proximity transceiver, and though Seven deactivated it; his systems adapted and created a back-up, attracting a Borg sphere to USS Voyager. (VOY: "Drone")