So I finally got around to calling Five Star yesterday. Talked to the sales manager, Nicole, who was very knowledgeable and helpful. She said in her opinion the most likely cause of this residue was the dyes in the Sani Clean attaching itself to beer stone in the bottom of my fermenters. She also said the dyes could not cling to the surface of 304 stainless if it was indeed truly clean.
To get rid of the beer stone she suggested an extended soak in a strong solution of Acid number 5. Which I will do but still don't plan on leaving the sanitizer in the kegs for extended periods.
In case your wondering how the beer stone got there in the first place, the previous brewer who I replaced was not at all fastidious in his cleaning habits. Though I can't physically see the beer stone it doesn't mean it's not there and doesn't surprise me at all. Cheers!