In reply to mtn:
Here's an article on the company that is using boots instead of towing using a plate reader. This was all I could find searching from my phone. The program is run by the NYC Marshall and it is to collect from people owing $350 or more in parking tickets. It is commonly referred to as scofflaw towing.
http://queenscourier.com/2012/city-scofflaws-to-face-the-boot/
The Ct. state police used to use all kinds of unmarked cars. Caveat was they were almost always a cocoa brown. Don't know if they still use them because I no longer have to go back and forth from Ma. to Pa. once a month to take my grandmother grocery shopping.
We have a handful of unremarkable unmarked vehicles in my county. No county tags, no antennae or anything, they're all old, kinda beat-up, seized/forfeited vehicles, the only common thread is that they are all domestic brands. You would never in a million years guess any of them were county vehicles.
As far as lights go, when my father was with the local volunteer FD he had a blue mini lightbar on the car. On his trip to Virginia he was pulled over shortly into the state and given a STRONG warning and nearly had it confiscated. He pulled it and left it in the trunk for the rest of the trip.