Well, it is known that thiocyanate forms silver sulfide under the correct condition. This was once used to sulfur sensitize emulsions. I have not seen any good analytical information on this though.
Formation of silver sulfide is like sulfur toning in that sense then if this is what is going on. Every time silver and thiocyanate interact, silver sulfide forms, gradually forming a black speck of silver sulfide and protecting the rest of the image.