A sensor ghost was a malfunction of the sensor array on board starships and space stations.
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 William T. 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: "Cause and Effect")
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")