Not UFO, but lightning can appear identical to sightings and persist for unnaturally seeming long times.

Have often observed UFO like orange to dark red flashes and glows as the warm summer ocean moisture ladened air stacks up to the north and east of us creating embedded glows.  The storms can be so intense they seem glow relentlessly.  Occasionally, patterns of glow would "ignite" and travel 15-30 degrees across ones field of vision well above the horizon appearing to be objects in motion and rapidly changing direction.  
We were mystified until we learned the glows were cause by energetic lightening storms taking place approaching the Sierra Mountain Range.  The storms embed in clouds that can be invisible as they blend into the dust laden air.  The lightening create lights and shapes that move and persist for many seconds at a time ... and when you witness it, it seems like minutes.
Witness elects to remain totally anonymous; provides no contact information.  PD