The 5D Mark III is a magnesium-alloy body.
That means to make it look black it needs to be painted.
If you rub any metal that has been painted for long enough against clothing, the paint will eventually wear away and show metal underneath.
The pic posted in this thread looks like the camera's been rubbed with sandpaper. Looks like a QC issue - I'll be careful with mine if something similar appears to be developing.
16-35L, 24-70L II, 70-200L II, 100-400L, 50L, 85L II, 135L, 24TSE, 40, 100 macro, 600RT x 4
I have more photo gear than I need. The blame lies squarely with Canonrumors