Black spots on MacBook Pro Case

I work with a Macbook pro and have it on a stand all day at work. When packing it up to leave, I sometimes (usually once a week) see one or more quarter sized black spots on either the top or bottom of the case. These spots look like someone rubbed pencil all over it. 90% of it comes off with wet rag but still leaves a small peice of it when I'm done. When rubbing it off, it smears around the case, almost like pencil lead like I said before. It is really odd and it's annoying since it happens frequently. Does this have to do with heat or?

I have looked around for the answer to this problem of these forums but only found instances where it happened around the track pad (mine does not) and where the spots are more like specs (mine are bigger).

If someone could shead some light into this issue and how to resolve it, that would be greatly appreciated.