Erm... we weren't expecting this.
There must be a logical solution. Perhaps it's a prop for when he throws his toys out of his pram over Europe?
Maybe he's practising putting on trousers to show who's really the boss in his marriage?
Or it could just be he's being incredibly thoughtful and preparing to clean up the mess when Ed Miliband sees the latest poll results?
To be honest, it probably belongs to his daughter, Florence, but that's a just a touch predictable and boring...