There is very little DOF with a scanner lens, all too evident by the number of people who make home made adjustments to get their scanner focused. Some are better than others, but essentially if there is some dust on the scanner lens it will be so far out of focus that it couldn't show up as a sharp line.

The OP should look for something that would scratch the film, and while the backing paper offers some protection to the base side, the emulsion still gets dragged across the film gate, either supported by rollers, or with cruder cameras right across a smooth painted surface. But it looks like a processing scratch to me.

Steve