Strange object in the sky

I'm a hobbiest photographer. I had my camera set up on a tri pod and was doing what is called photo bracketing. I took a series of 4 shots at different shutter speeds. The four photos were taken within seconds of each other. It wasnt until I got home and was processing the photos that I came across something odd in only one of the photos. At first it appeared that my camera lens was dirty, but when I zoomed in on the photo I found something very strange in the left hand side along with all the other clutter in the photo. Everyone who has seen the photo so far agree that its very strange.