I once had a similar thing happen. I was able to trace it back to the last rolls of film that I had developed, which I had dried on the reel in a hot-air dryer. Some of the emulsion from those rolls had stuck to the reels when I took them off. When I later developed more film, the emulsion on the reels decided to migrate to the film surface and deposit itself there. It was a weird thing, so I doubt that what happened here is the same thing, but who knows....thought I'd throw it out there.