Could be railroad dust as…
Could be railroad dust as opposed to brake dust. It was something very common back in the 80s when railroad cars were open, but it does still happen. A fallout remover will remove these spots. We used to have to do this to most cars to remove the coarse residue that was on new cars when they arrived at the dealership or they too would eventually show "rust" spots in the paint. The dust from railcar wheels would embed itself in the paint even though the cars were shipped with shipping wax on them, and yes, the other comments are correct - stainless steel does not corrode like regular steel.