Unfortunately an image has been built up in some quarters that stainless steel can be installed and forgotten. Perhaps it would have been better named Stain Resistant Steel. Too many stainless steel building materials, signs and fabrications are allowed to deteriorate, mainly through lack of a cleaning regime or by subsequent free iron contamination, with poor design sometimes giving a helping hand.
If the problem has not progressed beyond mere soiling, then a wash with soapy water, well cleaned off, will provide the solution. However if left alone, the deposits can, either on their own or by entrapping other atmospheric pollutants, break down the passive oxide layer and initiate corrosion on the stainless steel. The products of corrosion can be removed with acid based cleaning solutions but the actual site of corrosion may need pickling or mechanical polishing.
The process rejuvenating the stainless steel surface is commonly referred to as “restoration”, a service offered by Vecom.