Strange brown object over Clayton, California

On October 11, 2014, after a birthday party in Clayton (California), a person begun to go at his home. He observed the mountain before to take it on photo. Later, he noticed a dark UFO, no visible at naked eyes. What is it?

Witness statement:

After attending my niece's birthday party at Clayton Community Park in Claton, California opposite Mt Diablo, I went to the parking area to leave. I could observe the mountain side from where my car was parked. I took three photos of the mountain around 3:45PM. I did not see the object at the time. The horizontal lines on the pyramid shaped mountain were interesting to me. After downloading the photos to my computer I discovered the strange brown object in the third shot. The object looked like it was cruising along or hovering in place. It's difficult to judge as this is a still photo. I was shocked to see the object in the photo as I didn't see it while shooting it. I have heard that UFOs are often seen around mining areas.