What revealing mistake does Steven Spurrier make in the novel, All That Remains?

Scarpetta becomes aware of Spurrier when, late one night, she and Abby are getting into their car in a parking lot and he approaches to ask for directions. Scarpetta is suspicious that he did not actually need directions because he has a sticker on his car proving he is local, so she writes down his license plate number. This number leads Marino to learn they are stolen plates. Shortly after, Scarpetta connects the book store Spurrier owns to the murdered girls, Mott and Harrington.