Air is full of moisture.
Are you spraying any cleaning products near it?
Chlorine containing products can make even stainless steel rust.
OT, but when I wasd a student I worked in a lesuire centre, which had a big swimming complex and eating places etc. One day the "chemical man" comes and delivers the chemicals, basically using hoses which pumps into the system. Well he got the chemicals mixed up, and it ended in chlorine gas being produced and sent through the full air conditioning system, ending in a lot of people needing hospital.
Well, after it was safe to enter we had to go in and start the cleanup, anywhere there was metal, any metal at all, had a lovely coating of oxidisation, including every single thing in our stainless steel kitchen!!
http://news.bbc.co.uk/1/hi/scotland/1494040.stm
Chlorine = bad!