If you follow the stray light with a stick does it take you into the lens?
If you put black paper in the negative carrier and completely cover the opening for film. Do the lines go away as the negative carrier opening is blocked out?
If so, what happens if you show just a sliver of one edge of the negative carrier? Does one line come back?
I was thinking it might be reflections off the opening edges of the negative carrier. If the carrier is a full-frame carrier (like a filed carrier would be), then around the negative will be four (or eight) bright lines that can reflect around.