A bogey is a vessel which has been detected visually or by sensors, but has not been identified.
In 2373, the USS Hastings detected a stationary disturbance near the Cardassian border. New executive officer Patrick Ingrum wanted to take a look at the target himself. He ordered a dummy warhead fired from one of Hastings's fighters. The bogey exploded once the dummy was within 100 meters of the bogey, revealing that it was a cloaked Klingon mine. (USS Baldwin: "Wild Roses")
Part of the mission of the 10th Squadron of the Fifth Fleet in 2376 was to watch for and identify bogeys flying in the vicinity of the Cardassian border. Under the rules of engagement, if the vessel is identified as being a "warbug", it is to be considered a hostile vessel, also known as a "bandit." (USS Baldwin: "Blue Clear Sky", "Like the Rain")