Sorry... couldn't let the opportunity pass.
The fact that the guy left his phone and wallet in the bag suggests to me he was probably only intending to be a few seconds or so. I'd warrant that someone walking behind him has seen their opportunity, scooped up the bag before it has had a chance to settle and kept walking... around the nearest corner. The dangers of feeling too comfortable in a place.
Blue
EDIT: Forgot to mention that "no animals allowed in the pub" is pretty standard and although his were well and truly constrained and not visible, he was either doing the right thing or had been ticked off about it before. I cannot think of any other reason you would want to leave your bag outside.