-daytime triangular craft over Sedona

Late afternoon with a clear sky, my mother was taking photographs of the mountains east of Sedona from near the downtown tourist area of Arizona. A few weeks later when she developed the pictures a black craft appeared in two of the photos. It only appeared in the negative and the developed photo. She had seen nothing but the view of the sky and mounains nor heard or felt anything unusual when she took the photos. The object is a triangular shape with the corners "removed"; exactly the shape of the UFO reported by someone on this same website from the U.K.
[See in NUFORC database:
Occurred : 3/12/1996 17:30 (Entered as : 03/12/96 17:30) 
Reported: 8/31/2001 05:33 Posted: 3/4/2003 
Location: Clacton-On-Sea, ] 
The object is in a clear, blue sky approximately as high as a plane flying relatively low and about 20 miles away from downtown Sedona. An enlargement of the photo from the negative seemingly show two sides in motion - that is, a bit blurred, with one ( of the three sides) having a more defined line. I will send a copy of the enlarged photo as well as the smaller snapshot as soon as possible.))
Date of sighting is approximate.  PD