Right mate, a few things.
Yes, they could be seeing your exposed flesh, but if there's little to no light, I'd imagine they hear you coming before they see you, and that is what is sending them running.
Rat's eyesight is poor by comparison to their sense of smell and hearing, but also they rely a great deal on touch. If you watch rats, you'll notice that they generally use the same paths time and time again, and they usually run along side something. This is to reduce their vulnerability as much as possible. For example if a rat moves along a wall, it has the ground beneath it, and t