Girl having fun

[candide]

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.

[EduPerez]

The surface on that square is quite reflective, and there are trees and tall buildings around that area; I am more inclined to think in an optical effect than a defect in the capture.

[seamus finn]

So am I, Edu - there is no sign of a technical/sensor flaw on any of the pictures taken either side of this one. The camera, a 20D, is long departed the scene of the crime and with it goes the mystery.