"through a thin wite paper". The oil on the lens has been covered already, you would find that by visual inspection. Another possibility is that cold weather could solidify some fog on the lens. It would disappear when you brought the camera inside again.

(Vegard, det är lätt att det blir lite imma på linsen. Jag har fått det då jag skyddat kameran innanför jackan, dumt nog...)