Mysterious Dog Injury

I came home Friday night to find my Great Dane in bed with my husband. While this is not completely out of the ordinary, the fact that his foot was bleeding all over my pretty bedspread was. He had an almost two inch gash in paw that definitely needed stitches. We were at the vet the next morning when they opened the doors and almost $300 later, I have a stitched up dog at home that won't leave his bandages alone. I can't hold him down long enough to get the e-collar on him, which means I will be wrestling him down to wrap his paw every time he decides he sick of the bandages. It's still a mystery as to how he cut his paw open. No blood trail or evidence anywhere in the house or backyard. Guess we'll never know!