Whoops! When we got home after a short trip out yesterday morning, our neighbour told us she had seen our front door open. I think we must have forgotten to lock it on our way out (a dratted double-glazed door that seems a touch inconsistent about whether it locks automatically when you pull it shut behind you).
Fortunately, it seems she was the first to spot it and she kindly shut it for us. We will be more vigilant in future though!