They probably used equipment on (things that are not stainless steel) and the stainless, contaminating the stainless. ( Any cutting/sanding/buffing equipment you use on stainless steel should either be new or only used for stainless previously, iircc. )
It looks like surface contamination to me, which can happen when iron or non-stainless steel gets embedded into the surface of the stainless through abrasive action, like scrubbing a stainless sink with steel wool, or as some people have stipulated, iron dust from the rail carriers during transport.