A magneton scanner is an engineering tool with multiple purposes; it's used to degauss the transporter systems, and is one of the most thorough scanning devices aboard Starfleet ships of the late 24th century. A magneton scanner can degauss an entire transporter room in about five minutes.
In 2371, Voyager tries to scan for an alien life form moving from one crewman to another using a magneton scanner, as Paris suggested it was the most thorough scanning device aboard, but it is unsuccessful. (VOY: "Cathexis")
Later that year, B'Elanna Torres suggested to Chell that a magneton scanner would degauss the transporter room a lot faster then the micro-resonator he was using, but Tuvok told him it would teach him "patience". (VOY: "Learning Curve")
In 2373, when Captain Janeway was near death, she hallucinates that Torres used a Magneton scanner to search for her remaining presence aboard the ship, if there was anyway to help her. But this later turned to be an alien-induced near-death-experience. (VOY: "Coda")