Had to have a flat changed so it was the first time the little plugs over the wheel bolts have been removed. Surprisingly, the heads on all the wheel bolts were corroding - rusty whitish powder and pitted. The shop said it was probably from the boat trip to North America - salty air/ water? I find that hard to believe, but something got in there and caused it ... anyone else seen this ? Pull out a bolt cover plug (use the little tool in the tool kit) and check. Makes me wonder what other little corrosion spots may be hiding unseen.