Originally Posted by Ko.Fe.
Did you tried to wipe it off by fingers?
I use Kodak photo-flo and 2 well wetted and squeezed out sponges to wipe the film before hanging to dry. I've done this since I have developed film with no effects.
I think this is a "bad spot" on the emulsion, as I can not detect any actual scratches on the film. If it were a bad curtain, I think it would be on every frame, and worse on a well lit scene, and it is the opposite, worse on a poorly lit interior.