Intruder alert is an alert status on Federation starships and space stations. It can be declared at any time, by any member of the crew upon sighting an intruder on board. It can also be activated automatically based on predefined criteria, such as entering a restricted area. The alert is announced by audio and crewmembers are to take appropriate action.
In 2367, Captain Jean-Luc Picard tried to call an intruder alert when "Ardra" appeared in his quarters. (TNG: "Devil's Due")
In 2371, when the energy patterns of the Komar and Chakotay were forcing the crew of the USS Voyager to perform actions against their will, Kathryn Janeway initiated an intruder alert. (VOY: "Cathexis")
Rom activated an intruder alert which was detected by Damar after Odo failed to deactivate the alarms of a conduit leading to a EPS feed. (DS9: "Favor the Bold")
In an alternate timeline, Neelix helped repair the internal sensors aboard the USS Voyager after they had been seriously damaged due to repeated Krenim attack. He wanted to try several variations on the audio warning, but security chief Tuvok ordered him to keep the standard alert. (VOY: "Year of Hell")
Tuvok initiated an intruder alert and called for security to deck one when the two astronauts from the planet that Voyager was trapped above in orbit transitioned into Voyager's timeframe. (VOY: "Blink of an Eye")
In 2374, the crew of Voyager initiated an intruder alert when they discovered that a member of Species 8472 had penetrated their hull and invaded the ship. (VOY: "Prey")