To me, a closed door - locked or unlocked - is code for "do not disturb". If a door is open or even partially ajar, I consider the room vacant and ready to use, unless I hear someone on the other side of the door. I think it's pretty weird that I have to knock at a non-closed door to determine whether I am permitted to enter. My main gripe is that my family is sometimes naked or at least indecent behind doors that ought to be closed. For me, those encounters aren't just gross; I find it hypocritical that my relatively puritanical family can be so shameless around the house (unless they have company over, of course).