The police find the broken rear number plate of the Ford Sierra car that knocked the old man over. But shots of the car before the accident show that the car's rear number plate was already missing.
Incorrectly regarded as a goof.
When the Ford Sierra is driven recklessly without a rear license plate, but discovered at the crash scene. DI Frost questioned how conveniently the plate was there, and whether the retaining screws had been found. The SOC Police Officer replied that they hadn't found any, which would indicate that the plate was not on the car and intentionally removed. Then deliberately left at the crash site to implicate the owner of the Sierra.
Someone who was not Chief Superintendant would not have master keys to the police locker room.
Much is made of Roger Massie's car losing its rear license plate following the hit-and-run, with Frost deducing it was deliberately left behind. However, before the old man is run down, the car crashes into other parked vehicles and loses its front bumper, including the front license plate. Nothing is ever said about this nor does anyone attempt to investigate that part of the incident. As such, the issue of the rear license plate being left behind would be moot, as they would have the front license plate anyway.