Lensrentals noticed that all of their D600 rental units needed sensor cleaning after being used. What is even more interesting is that all spots seems to be located around the same area – the upper left portion of the sensor:
Lensrentals did some additional investigation and found that “D600′s shutter curtain opening seems a bit larger than the other Nikon cameras with a bit of a gap around the shutter curtain. It may well be the shutter movement is pulling dust onto the sensor”: