If a door is placed directly against a wall as in the door opens out towards you (I found this while making an equipment locker but it does not matter what is behind the door but that might help with understanding). If you open and close the door a few times particals effects are visable and the room starts to flood at a rather quick rate (as in it looks like it floods at the same speed that the room would flood if an external door was open). I am unsure if this happens with all doors but I have tested it with hinged and sliding doors and after opening and closing a few times it always flood the room.