It could have been some minor water contamination, it could have been insufficient cleaning after fabrication and polishing.
It doesn't take much to get this staining on 304.
I suspect that it wasn't pickled well enough after welding, and then it should have been passivated (cleaned to remove residual iron in the surface) after polishing. That either traces of oxide were still embedded in the surface or that the Cr depeated material from just under the scale had not been removed.
= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed