As I was reading, I was thinking you had lens fungus/mold. But you checked for that. Also...elements clean. Have you taken a shot or two, even around the house? How do they look?
I have the Mk II version. There are felt like materials in between each element against the sidewalls. I suspect what you are seeing is the material itself or if you see something sitting on it, perhaps the felt doing its job and trapping dust.