Canon S95 - weird spot in middle of photo

Hi all,

I've been using the Canon S95 for a little more than a year as a subsitute for my D300 for places where a DSLR is not convenient. I'm more or less happy with the results from such a small camera but there's one thing that's really annoying me and that's the effect in the following photo. I've thrown quite a few shots in the trash can because of this round blurry effect in the middle of the photo.

The effect decreases if I zoom in or pick a smaller aperture. It's always a round shape in the middle of the photo. It's not grease on the lens as the before and after photo's are mostly fine. I do get it mostly when I take the camera out of my pocket in a hot environment.

Has anyone experienced the same problem and know what causes it? Is this lens flare? Is something in the lens/sensor fogging up? What can I do to prevent it?

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Chefke