I just find a bad bad thing, I noticed in high ISO shots (2500+), still to check at low-mid ISO.
I attach a screenshot madre with My phone of the screen, where I opened raw file in camera raw of CC15.
I see a lighter square in the middle of the image, too.
Shot with XF 14/2.8 at 2.8, ISO 4000, 30", with cap on, NR off. These where the conditions where I first saw the issue.