Who Goes There??: The proper way to announce yourself to a bathroom

I was just in my company's restroom when all of a sudden I hear this weird noise which could either be interpreted as someone knocking on the door with a stick, something going wrong with the heating system, or a bomb. Maybe. I sit still, and it comes again. I assume it is the former option and say meekly (in case I'm wrong and someone walks past and thinks I am talking to myself in the bathroom), "occupied!" Then it comes again, this time with a "Helloo?" like I am one of those miners stuck underground and the rescuers want to know if they're getting close.
Jesus! Can't a girl pee in peace?? "I'LL BE OUT IN A MINUTE!!"

There's got to be a better way of determining if a public restroom is occupied than rapping the door with a stick. Presumably a mop handle. For instance, one could knock and say, "It's the janitor!" and then, should anyone be in the bathroom, the occupant could reply "I'll be out in a minute!" instead of sitting and wondering what the fuck kind of noise was that?! and crouching in case it's a bomb threat.