When Lenny and Helen stop at the diner for some food, there are different cars parked in front of the diner. There is a dark color car parked with white cars, then it appears all white cars with one two-tone car at end with white body and dark roof,which was not there a minute earlier.
At the end they show the license plate on the patrol car. It is a standard passenger car plate, not an exempt plate that is normally issued to government/police vehicles.
When Helen and Lenny Frazier are leaving their rental a reflection of the boom microphone operator can be seen in the rear passenger windows of Leeny's car.