Emergency transponder

From Memory Alpha, the free Star Trek reference

An emergency transponder (or command transponder) is a portable device, containing its own frequency and code, used to transmit emergency signals.

In 2365, Lieutenant Worf gave Commander William Riker a modified transponder prior to the Commander's transfer to the IKS Pagh. Citing it as a "security precaution," Worf had slightly altered it to transmit an omnidirectional signal and an emergency call.

Later, while aboard the Pagh, Riker removed the transponder from his boot and actived it, sending a message to the nearby USS Enterprise-D. The captain of the Pagh, Kargan, questioned if the device was a weapon, then demanded that it be given to him. Moments later, he was transported off the bridge of the Pagh and onto the bridge of the Enterprise. (TNG: "A Matter Of Honor")