Spheric-shaped UFO over Denver, Colorado

On May 16, 2014, a person was vacationing in Denver (state of Colorado). During his holiday, he took photos. But later, he noticed a mysterious spheric-shaped UFO on them. What is it ?

Witness statement:

I was vacationing in Denver, Colorado of last year. I decided to take a picture of the scenery & not the object it's self. 1-3 weeks later I started browsing through my photos. That's when I noticed what seemed to be a objected in the sky. I tried listing off the possibilities of what it might had been. My first two assumption was that it could have been simple the moon or a air balloon. But to me it just didn't fit the two characteristics. So I then went on to assume the possibility of a spot on the window or on my camera lenses. I'm truly lost on what this might be. Hopefully I'm not just naive of something so explanatory.