Speaking generally, i.e. not about this photo in particular, as a photographer I'm always puzzled by the "didn't notice it till later" pics. If you're taking a walk around the lake, you're looking here and there with no specific purpose and it would be easy to miss something. But when you're taking a picture, you're consciously looking at what's before you with the goals of obtaining a pleasing composition, ensuring the pic is in focus, etc. So it seems odd to me that people don't see things that later show up in their pics.

With that said, it's a little more understandable in wide-angle shots like this one because there's a lot in the scene, and especially so if it was a digital camera with a back viewscreen as opposed to a viewfinder.

If I had to guess on this one, I wouldn't rule out sensor dust. There's either another flying object or a speck of sensor dust above and to the left of the mystery spot. It's especially evident in the full-res version.