Green’s Dictionary of Slang

(US police) an unmarked police car.

1990		(con. early 1950s) J. Ellroy L.A. Confidential 3: An LAPD unmarked.
1990		S. Morgan Homeboy 58: Joe stared at the unmarked.
2005		R. Cea No Lights, No Sirens 132: We sat in our unmarked across the street from the Tuff Gong.