"Riddle Me This!"

someone stole the key while you were sleeping,

While you are inside the storeroom, the murderer enters your home and switches the lock (which is open at the time) with a padlock of his own that looks identical--but to which he has the key. Crucially, he keeps your padlock open, as he does not have the key for it and thus cannot open it once it's closed. While you're asleep again that night, he re-enters your home, opens the padlock (his padlock) with his key, deposits the body inside the storeroom, and finally locks the door again with your padlock, which he has brought along with him.

Thus the room is sealed up from the inside and the padlock undisturbed.