Well, maybe I should not be jumping to conclusions, but that sure looks like what it is from. Why it happened only once, I am not sure.



It is so hard to remove that I thought it might have been from the casting process or an application of strong wheel cleaner, but the exterior chrome would show some damage, and it remains in fine shape. Anyway, here are some pics to show you what I am talking about.



They all have a drip-like pattern, starting at the centerline of the wheel and then migrating towards the (lower) inner edge, which suggests gravity was at work (hence the dripping-from-the-calipers theory), and resisted all removal attempts, including scotch-brite, steel wool, wire-brushing and sandpaper.