I do see the smudge on one or two images. My initial reaction is it looks like fogging of moist warm air on a cool lens. It could also be water drops or something similar that is far out of focus.
One idea is to check the sensor and check both ends of the lens. While unlikely, its possible to have something on the end of the lens inside the camera. I've seen a test with a water drop sized post-it note showing up as a soft blur.
One possible reason for not seeing the problem later is a water droplet could have dried.