Well, it's in the direction the film moves (unless you've been holding the camera sideways! ) and the back, non-emulsion side should be protected by the backing paper (unless you had some 220 sandbagged). There are two very small rollers on the back portion, one above and one below the film gate. They are pushed toward the film by little leaf springs at the end. My guess would be one of those has some bits of ?? on it, or maybe for some reason isn't turning -- or there is a loose piece of grit wobbling around that area.
I think scratches can appear lighter or darker, depending on how deep and even the shape of the groove.
That's what I was originally thinking, but don't those rollers go down the middle of the film? Also, wouldn't they roll (& rub if they weren't turning) against the backing paper?