OUT OF SERVICE
NOT IN SERVICE
The sad truth is that the general public would not know the difference. Both of those could be interpreted as a real police vehicle that has been sidelined temporarily for a variety of reasons. Here is a CHP Vic pulling a trailer for a motor, and the car is marked "out of service".
Personally, I kinda like this variation:
NO LONGER IN SERVICE
...which I think is closer to accurate with very little chance of being misunderstood. If I had a marked car, I would probably create a magnetic sign that read:
SHOW CAR
NO LONGER IN SERVICE