Those spots look like drying marks to me.

The sky is VERY bright meaning the film has gone almost all the way to fully exposed. The spots are appearing even darker meaning if they were optically produced, the sources were very bright. Lens flare won't leave that kind of pattern and neither does light leaks. I'm thinking drying marks which should be visible if you look at the film carefully.

Are you able to view the film under a microscope or at least a good magnifier and see what it looks like?