I have a similar problem with my little nikon cp4500. I got well and truly caught in a thunderstorm, and the camera, although in a case inside my rucksack, was subjected to a certain amount of humidity and, I'm guessing, condensation. This has left a small circle somewhere on the sensor or some internal surface - it looks as though there's a deposit where the moisture has dried. Like your camera, the 4500 is sealed and there's no exposure of internal parts as there is with dslr cameras.
I've long since bought a dslr so haven't got round to sending the cp4500 off to a nikon repair place, but your post has reminded me. I will let you know what they say...
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Sharon
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