I agree that it doesn't look like scratches. When tilted back and forth in a good light source, there's no indication of actual surface damage that would be typical of a marred surface. I also agree that it doesn't look like oil or other residue, and certainly didn't behave like it. I wonder if it's something under the surface between the layers on top of the sensor? Could it be pressure induced?
I suppose Fuji will have to tell me. I assume they've seen it before, I can't be the first person who's experienced this. Normally I would hold on to an item longer and try to figure it out myself. I'm usually quite adept at delicate work with my hands, but my camera is something I just can't live without, and the more time I spent pondering the longer it would take to ship and fix. A fuji service rep quoted 2-3 weeks after arrival just to look at it. Oof!