Could it be a ghost image from less than total formatting out of a previous image?
It doesn't work that way. A partially erased digital image file can't appear as a ghost in another image.
I was convinced that it was crud on the sensor, but now I'm leaning towards shadows. But it wouldn't hurt to get a sensor scope and check out the sensor anyway. In my opinion, a sensor scope and a good sensor brush, such as the Visible Dust Arctic Butterfly, should be standard equipment for every serious DSLR user--if you don't already have them.