A warning buoy, also referred to as a warning beacon, is an automated device that maintains a position and transmits 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. (ENT: "Observer Effect")
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. (TNG: "Identity Crisis")
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. (TNG: "The Outcast")
In 2375, Captain Kathryn Janeway had warning beacons placed around the telepathic pitcher plant, a bioplasmic organism that feeds on starships. (VOY: "Bliss")
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. (VOY: "Memorial")