I think it is probably from either end of the camera back or a canister window if it has one. When I ran a test film through a Leica R4 recently it produced some very similar looking fogging. In that case the foam around the film window was rather crusty (and in hindsight, obviously so) but the door seals could use replacing, too.
Some cameras are much more particular about back light seal condition than others. Eg. I have never bothered to replace the ones in any of my SRT-101s because I read that they are not critical and indeed, zero problems to date. Classic German cameras can be hard to beat because they frequently did not depend on felt or foam pieces to exclude light entry around the film gate area. You have not told us what the camera in question is but my guess is it is Japanese or late(ish) German and that serviceable back seals are fairly vital to effective light exclusion.