I meant to mention this one.
Cliffs notes: unmarked Silver Crown Vic in eastern MA.
Memorial Day weekend, I was heading to Worcester in the Subaru. Saturday afternoon, tons of idiots on the road, like every summer weekend. (If you don't know how to drive on a highway, then don't do it when you travel!)
I'm on 495-S in the left lane, following a bunch of cars at speeds varying between 55 and 80mph.

I see a silver Crown Victoria with MA passenger plates (##x x##) and no window tint on an onramp. He casually makes his way to the left lane behind me. Traffic slows down, speeds up, slows down, speeds up, and of course the space between cars moves like an accordian. There were points I was blatantly tailgating the minivan in front of me as traffic slowed and I used engine braking. When traffic lets up, the minivan and I accelerate to 80+mph, then traffic slows, etc.
The Crown Vic is behind me this entire time doing the same thing ... tailgate, let up, tailgate, let up, just as I am doing with the car in front of me. This goes on for 8 or so miles. When traffic slows again to <60mph again, I'm tailgating the minivan, he's tailgating me. All of a sudden he turns on the siren and blue bubble on his dashboard.
I swear my heart stopped. It definitely skipped a few beats. I probably also cut off a car as I quickly moved into the middle lane.
The minivan moves to the middle lane, and a few cars before that, and the Crown Vic takes off. Maybe he was responding to a call, or maybe he was just sick of speeding up and slowing down.