David, Poco,

I think that's the clue: Bright lights just outside the film frame will 1) be far off the optical axis, and 2) have a good chance of reflecting off any internal surfaces in lens and camera.

I have a beautiful example of this, where the sun was just outside the frame. Since the film was Konica IR 720, I was also able to prove that multicoating is not optimized for IR light... I would never have thought that there were quite so many surfaces in a Zenzanon PE 40mm...