I've used a very wide array of urban bathrooms over the past several decades. People are not smart. People can and will keep trying to open a locked door and need reassurance that someone else is in there already shitting. Silence does not accomplish this; "come back with a warrant," which I also saw, is going to be my new go-to as well, replacing the tired old "occupied" - which, while it got the job done, I now feel was too dry.