These fractures can be missed on the x-ray either because they are obscured (as in this case) or not looked for/recognized.  They represent an avulsion at the origin of the bifurcate ligament joining the antero-superior calcaneus to the navicula and cuboid.  If not treated by immobilization or fixation they can produce long term foot pain due to non-union.  The marked surrounding soft tissue swelling in an indication of the trauma involved in producing such a fracture.