Those would be real unmarked cars, not just a different form of standard police car.
The police in the unmarked car were easy to avoid.
"And I saw you get out of your unmarked car."
The president and the two agents drive the unmarked car to a woman friend's house.
He looked down the block just in case an unmarked car was watching.
By way of the back stairs and an unmarked car.
Three other men watched the apartment window from an unmarked car.
Then he has to stay awake in the unmarked police car.
He was stopped by four men in an unmarked car who dragged him away.
So for the next two hours, the agents, in an unmarked car, followed the man.