It is my understanding you can another officer to the scene -- you don't even have to roll down your window. Or maybe that's just for non-uniformed officers......

Drive to a well-lit safe area and if in question, respectfully request another officer be brought to the scene. I don't see why they wouldn't deny it -- but if they're a legit officer expect to get raped in charges for every possible thing they can find for questioning their aaaathoritaaaay.

Anyways.. to be on topic...
These different make/model unmarked cars we see -- i doubt they buy them new or anything, and don't see how it's spending taxpayer money or what not. They are most likely repoed or seized.