Oh what a shame! I have made it a rule that I always, always go out. If the dog barks, there's something there, so it's coat, boots and torch. Sometimes if its a nice night, I end up just going for a walk at 3 am. Mr fox and probably some smaller preds try your shed every night. They wait for you to slip up, for the door not to be quite closed or as I did years ago, for you to shut one out by mistake! It's a horrible thing, but the majority of poultry keepers I know have lost birds to wild animals at some time. Sorry to hear this though, not nice.