I've seen similar marks on my .380 brass, as well as a few other calibers. Here's a typical shot of what the .380 brass looked like:
I'm pretty sure it's just a result of shooting relatively low-pressure loads. The case doesn't expand enough to seal completely against the chamber walls, so a little bit of the combustion gases work backward between the chamber wall and the case. Like you, I didn't see any damage to the chamber itself. Subsequently, I bumped up the powder charge a little bit, and the "burn" marks went away.