Is it on all 4 edges of the frame, or just 1-2?
If it's on 1-2, that suggests a light leak at a high angle that it would 'get under' the frame edge (when scanning, I've found shots taken with WA lenses have larger frames than normal lenses, because of the higher incident angle).
Light leaks do tend to be very uneven though, you should see some kind of flare or something in the image.
But if it's all 4 frame-edges, then I'd be thinking pressure-plate problem, no way a normal light leak could be that even.