A sensor ghost was a malfunction of the sensor array on board starships and space stations.
In 2367, while en route to T'lli Beta, the crew of the USS Enterprise-D observed a series of strange patterns that appeared and almost immediately disappeared. Commander William T. Riker recommended running a diagnostic on the forward sensor array, suspecting that the observed readings may have been a sensor ghost. The "ghost" was later determined to be caused by a group of two-dimensional beings; the lifeforms existed in only two dimensions, which caused the unusual readings and made it difficult for the Enterprise to initially observe them. (TNG: "The Loss")
In 2369, when Lieutenant commander Geordi La Forge's modified sensor array began experiencing glitches due to the interference of the solanogen-based lifeforms, Commander Riker said it would probably be best to keep the whole array offline until they had time for a closer look at it, as they didn't have time to waste chasing down sensor ghosts. (TNG: "Schisms")
During the Klingon-Cardassian War, the Cardassians often employed sensor ghosts and holo-projections as decoys against the Klingon forces. (DS9: "Soldiers of the Empire")
When Seven of Nine traveled back in time to 2372 aboard the USS Voyager, Harry Kim detected some strange chroniton fluxes on deck four. Janeway told them to erect a force field around the area, and Chakotay asked her "is this really the time to be chasing sensor ghosts?" (VOY: "Relativity")