interesting. thanks for your help everyone. I am still wondering - the foggy area is offset from the image which clearly means the unwanted light is coming in at a more extreme angle than the light from the lens. This makes sense vertically, as the fog does not extend to the top of the image, and extends beyond the bottom.
However, it is wider than the image area on both sides, which would mean that light is coming at an extreme angle from two different directions but creating a fogged area with a uniform (and sharp) top and bottom border. Can that be explained by flare?