A warning buoy, also referred to as a warning beacon, is an automated device that maintains a position and transmits a repeating message that contains a warning about something nearby.
In 2367, Captain Jean-Luc Picard ordered a warning beacon to be placed in orbit of Tarchannen III to warn others of the [[Tarchannen III species|invisible species there. (TNG: "Identity Crisis")
In 2376, Captain Kathryn 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")