Another strange artifact and a new camera

On Christmas morning I got a digital camera and printer from my wife. I'm new to digital having always used 35 mm cameras in the past. I read enough of the instructions to take pictures and print them. One of the pictures that I snapped was of our Christmas tree and part of the parlour that it is standing in. On the right wall of the parlour appears a disk shape. just like the ones I snapped on Halloween night. The Halloween parlour shot (taken on an old 35mm camera) showed the disk to be on the back wall. The Christmas shot shows a disk on the right wall of the parlour. Two different types of cameras, same room, months apart, both showing a strange disk shaped artifact but each in a different part of the room. I really would like to know what is causing these abnormalities. If natural, I'd like to correct the problem. If not natural....then what?????