The carbon content you show is certainly low. Without more details on the application, here is a shotgun approach of things to look into. Some are unlikely but all are at least possible:
Is there any chance of other alloy content that is relevant? The other interstitials that can tie up Cr include N, B, and O. A heat treatment in a nitrogen atmosphere could result in some nitride formation.
Chloride SCC (and probably all of the other causes of SCC) is temperature dependent. Was the service greater than ~150F / 65C? Did conditions result in a concentrating effect such as drying of a solution on the surface?
Were there additional 304SS components exposed to the same environment that did not have attack?
Were there other species than sodium chloride present? For instance, MgCl2 is much more aggressive than NaCl. Liquie metal embrittlement from Hg?
Was the surface possibly in a different condition than expected, for instance a too-aggressive pickling solution that attacked the surface before service?
Has the presence of other intermetallic compounds been ruled out, such as sigma phase?
Was the actual service environment the same as design, for instance were other corrodent species or unexpected temperature excursions documented?
I hope these questions are useful.
Paul T.