If a car is behind you, would you necessarily be able to tell if the driver is in uniform?
Imagine being stopped by an SUV on a very remote road, and the driver gets out and he's dressed in all black military fatigues.
My first instinct is not to look for my insurance forms, it's probably to figure out if I can get the hell out of there.
Of course, given what was going on I'm sure they had an idea what was going on.
It also appears likely at this point that the driver of the other vehicle was a LEO plant. So, his reaction was different because he was in on the setup.