I saw this sort of thing happening on I-20 a few months ago, and I also wanted to make a call, but I was away from the internet that day, and I actually forgot what county it was.

Between Atlanta and Augusta. Maybe Siloam area. I tried to take a picture with my camera, but it too came out way too blurry to make out the markings or license on the car. I didn't have my FLIP video camera readily available, either.
The car was going way too fast for me to even try and speed up to their speed. Passed me like I was standing still and I was going 76+MPH. He/she was easily going 100+ MPH, no lights on, no siren, passed me on the right, cut in front of me (as there was a car in the right lane in front of him/her) and was very quickly out of sight. It was also at night, completely dark. The vehicle had county markings on it, but it was an SUV, not a patrol car. If my memory serves me correct it was a Ford Expedition.
I'd like to do something about this as well when I see it. I think the next time I see something like this I will try to. It just sort of rubs me the wrong way.
I totally understand if LEO are going to a call, though, I totally understand that, but I would just rather them use lights, if possible the siren, and I totally understand having to pull up to a crime scene with none of them on, if the situation calls for it. That still doesn't mean they have to have them all off on the way to the scene while traveling at a very high rate of speed.