Object appears on one digital photograph but not the next frame taken seconds later.

Sighting Report:   Last week I was reviewing some photos I had taken a few weeks ago, on April 18th. I noticed a large speck in the sky on one shot. 
Using a computer to enlarge the area by 1000% I can see it appears to have a flat bottom and top of different circumferences.  The left side of the narrower upper section appears to either be reflecting or emitting light. The object does not appear on the next photo, which was taken less than two minutes later.
I was facing Southeast from the north bank of the Schkulkill River, near the Art Museum.  The building is across the river.  
I didn't notice the object when I took the photo or when first reviewing them. There are no other similar objects on any other photos.
We have reviewed the photos, and we do not know what the object is.  It is very distant, very small, and blurry.  We cannot rule out the possibility that the object might be a bird, perhaps a pigeon, but that is only a guess.  PD