Omg!! I had the same police car glitch, how did you do this? I've posted it several times here already, and also on Driver-Dimension, but nobody knew.
I was always wondering how I had done that, but basically it always happened when I blew up a secret car or something in Istanbul, or anything related to one of the secret cars. When I selected any car then I would start in the police car and I would spawn where you have spawned.
I'll see if I can try to find my old post again, actually I posted this again not so long ago, maybe I find a bit more info about what I did