I can throw something out there, but it's really just an educated guess.
From what I understand, the process of making steel stainless involves adding a thin layer of iron oxide to the surface. Yes, that would be otherwise known as rust, but we're talking really thin layer. This works because the whole exposed surface is already oxidized, so the reaction can't proceed any deeper. But when you scratch stainless steel, like with a sharp knife, you remove that surface layer and the exposed steel starts to oxidize.
I suspect that you're right about the sulfur compounds. A lot of taste and smell chemicals are sulfur compounds. Sulfur is not terribly electronagative, meaning that it doesn't hold onto its electrons (or steal them from other atoms) very strongly. It's not weak, just that it's such a big atom (relatively speaking) that the attraction between the nucleus and the outer electrons (valence electrons) is weaker than its smaller cousin oxygen. So in the tug of war for electrons between the two, oxygen wins.
I suspect that the iron oxide on the surface of the steel acts as a catalyst to oxidize the smell molecules. But like I said, that's just an educated guess.