I once had the "do not disturb" sign on my door when I wanted to take an afternoon nap, and housekeeping hadn't cleaned the room yet. They didn't knock...instead they called the room! I'm sure there are people who forget to take the sign off the door when they leave the room - but that's their fault if the room doesn't get cleaned. In my opinion, a "do not disturb" sign ought to mean leave my room alone!