Warning buoy

A warning buoy, also referred to as a warning beacon or a marker beacon, was an automated device that maintained a position and transmitted a repeating message warning about something nearby.

In 2154, Captain Jonathan Archer left a warning beacon in orbit of an M-class planet that hosted a Klingon garbage site to warn others of the very lethal silicon-based virus one could find there. 

In 2365, the USS Lantree's marker beacons were activated by Lieutenant Worf as part of an ongoing medical quarantine. 

In 2367, Captain Jean-Luc Picard ordered a warning beacon to be placed in orbit of Tarchannen III to warn others of the invisible species there. 

In 2368, after the pocket of null space was mapped in the J'naii system, Worf devised the plan for deploying warning buoys around the pocket. 

In 2375, Captain Kathryn Janeway had warning beacons placed around the telepathic pitcher plant, a bioplasmic organism that feeds on starships. 

In 2376, Captain Janeway ordered a warning buoy to be placed in the Tarakis system, to warn others of the synaptic transmitter that was in use as a memorial there.