An UMP (aka Ghost Cop), is a Cop Car that is not marked as such. It is specifically made to look like any other vehicle on the road in the hopes of catching a 'lawbreaker' unawares. Usually this means speeders, loiterers or vagrants.
Although the legality of using an unmarked police car has been questioned constitutionally since their inception (2 States in the US have deemed it 'entrapment'), that has not deterred police agencies from employing them and in 1998 in the United States, 15% of all citations for moving violations were given by officers utilizing unmarked police cruisers/undercover cars.