[johnsherwood]

Just a thought, but its probably pretty cold in Sweden this time of year.......any chance this is just condensation from going back and forth from warm apartment to cold balcony and caused a little fog?

I'd say if not condensation then I'd try the same test, in the same way ( temp/time outside and inside ) with the K100 too and see if you get the same result, if yes then its lens/fog/condensation...........if no then as others have said likely AA filter or sensor require cleaning.

I'd let someone else clean either the filter or the sensor rather than try myself......

Good luck,
John