Reading this reminded me of a similar occurrence I had a couple years ago with 10 gauge barrels. I got the gun out to clean it and when I looked down the left bore there was a line running the length of the barrel. I started thinking the worse. Running a bore snake didn’t phase it. I was wracking my brain as to what it could be when I suddenly remembered the last time I cleaned it. I scrubbed the bore with #9 and only wiped out the right barrel. I ran out of patches before I could wipe out the left. I let the gun sit overnight on the bench and
the next day put it away without giving another thought to it. I believe the residue from the #9 pooled and dried in the barrel as it laid horizontally. The drill and pad routine did remove it much to my relief.