I had a three tube carbon Trek come in for a flat repair and clean up the other day. I noticed some nasty gouges on the non-driveside chain stay. Upon closer inspection, I found it was actually worn through!
This was beyond where the crank arm could have contacted it. Maybe a case of bad heel rub? Who knows, but this frame looks like it came in contact with a grinder to me!
Here is a shot of most of the chain stay to the drop out. It looks like a metallic dog chewed on it some.
Sorry for the out of focus shot, but you can see other things in this image.