I've never done it, but part of the reason is that it can be hard to reach the door and knock from the place where I might sit (or stand, as I'm male) in a bathroom. That varies from one bathroom to another, of course.... in Japan, we often knock back without saying anything.
Do you do the same in the U.S. ...
Not only do we not do it, but it could be considered quite rude and insulting and I suggest you not do it over here ... particularly if you're in the loo at a truck stop and it's a big, burly truck driver knocking at the door.Thank you, all. It really helps.
By the way, in Japan, we often knock back without saying anything.
Do you do the same in the U.S. or Britain?