Here, in the western U.S., we have the same problem with coyotes. A neighbor, (3 miles west) had a work boot 'stole' recently… said it wasn't the first. I think you're on the right track about the fox possibly eating the leather, although my neighbor surmises they like the 'salt' taste left from the human wearing the item.
Fortunately, our work dogs that live in the barn usually are less than friendly when performing greeting duties to the coyotes… One early morning not too long ago, heard a ruckus early in the morning and on arrival at the scene near the barn, one dog still had the errant coyote by the throat and another by a hind leg stretched tighter than a fiddle string… end of story, as they say.