This expression refers to a cop's intuition about impending danger, about whether or not a suspect is guilty, about whether someone's lying, about hunches cops have regarding cases or people. The term is used by Lyons and Truzzi to refer to something akin to "psychic powers" used "to solve crimes." (p. 11)
See related entry on psychic detectives.
further reading
Lyons, Arthur and Marcello Truzzi, The Blue Sense: Psychic Detectives and Crime, New York: The Mysterious Press, 1991).