We've only recently put a lock on the bathroom door, and that was to put visitors more at ease. I even switch off the webcam now if I know they are a bit uptight about that sort of thing. Seriously though, only just put a lock on the bathroom door. Rule for 30 years has been, if the door is closed, there's a good chance there is someone in there. If the door is closed and you don't know where I am, you are more than welcome to burst in, but I may be having a shit and / or a wank. We all do it, I'm not going to pretend I don't. Knock if your nervous. But yeah, we've had a lot more house guests lately, so we now have a lock.