"The door had the receipt stuck in it, so it couldn't have been used. He also couldn't have gone through the window, because you can't lock it from the outside, and when Aunt Eva visited the study, she checked to make sure it was locked. So the window also wasn't used. At that point, the study really was a closed room. We have to accept this! However, the study didn't remain a closed room forever. Once the seal created by the receipt was removed, it'd be possible for someone to escape by the door."