Youíve shown a short somewhat irregular line. Obviously, itís not an air bubble.
Since the line is dark and you described these asĒ little spots of less densityĒ, I presume the negative has been reversed to positive.
Given the shape, I wonder about the size. Might it be about the size of a tiny fiber from clothing?
Iím thinking not of threads, which are relatively huge, but rather the very tiny fibers that we can sometimes barely see in beam of sunlight on the surface of, say, a cotton or cotton-polyester blend shirt. I canít see them unless Iím wearing reading glasses.
Iím wearing such a shirt now and can barely see many fibers that Iíd estimate are 0.0001Ē or less in diameter and possibly 0.020Ē or somewhat longer. If it werenít for the beam of glancing sunlight I probably couldnít see them at all.
Film can be much like a magnet while weíre loading holders in the dark. That will draw almost any particles to it.
The fibers Iím seeing are almost translucent. Something like that might well correspond to what youíre seeing.
If there were only few here and there, the idea of stray fibers might make sense.
If there are a great number of them, then Iíd reject the idea.
Possibly someone else has a better explanation.