Also used in the sense of UFO flap, i.e. a group of U.F.O. observations that are close in space and time.
For unknown reasons, it often happens that a lot of independent U.F.O. observations happen in a place during a given period of time that may be of weeks or months, and then disapperar. Such flaps may happen around military devices or maneuvres, or around nuclear power plants, or for no apparent reason at all. Such flaps happen all of the time, just go to any ufology web site and find out how many are witnessed all of the time.
Famous flaps in ufology were the ones in Hessdalen, Norway and the ones on Monte Orsa, Italy in the 70's.