From what I understand, the facility where the county had their marking and striping of their patrol units had brought one of them there for some work. One of their workers last day on the job was on that day. Supposedly, the guy was not a happy camper the entire time he was on the payroll, so, as a matter of getting even and the last word in, this is how he decided to exact a little bit of revenge on those around him.
If you notice, it's marked on the "Passenger" side only. It was done that way for a reason. Many officers never do a walk around (Like they should) of their unit prior to going out on the road. Needless to say, the Crown Vic was driven for an unknown amount of time before it was discovered.