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jims said..
I'm not a welder, though I've done a little bit of welding in the distant past, including on Stainless steel. It is entirely possible for a weld on Stainless steel to rust (it befuddled me the first time!) IIRC, it can come from just the heat of the welding process (destroying some of the rust-resistant properties of the stainless surface), can come from impurities in the filler material (ie, iron content) or even the environment, and can even result from cleaning the welding area with a regular steel brush. (I'm sure there are more that an actual welder could expand on, but those are the ones I remember learning about at the time.)
I have a lot of 303SS booth weldments made at work. They "passivate" the welded areas to remove the surface iorn that has been freed from the welding process to prevent rust. The solution contains nitric acid.