Other than H-1, I am not aware of any steels that are truly stainless. So even steels dubbed stainless can still rust and stain, they should be called stain resistant. Some peoples sweat will cause rusting, other it won't. It comes down to the body chemistry of the individual. But to answer the question, sweat could be the culpret.
Just because you can't see the difference doesn't mean it isn't there; it just means you can't see it.
Every cutlery alloy is good for something. No cutlery alloy is good for everything.
Order of Kai #0566