The Bathroom

So I've recently noticed that whenever someone knocks on the bathroom door, I respond in an odd way. I don't think I've ever really given it much thought, but whenever I'm in a public restroom and someone knocks on the stall I ask, "Yes?" As if I didn't know why someone would be knocking on a stall door, as if this complete stranger was knocking so they could strike up a conversation. Wouldn't the more acceptable response be "Someone's in here," or "Occupied,"? Yet I consistently answer with some sort of a questioning acknowledgement. It makes me sound like a hopelessly confused dog; why would someone do such a thing? I can only hope that one day, I'll either respond to the knocking correctly, or the mysterious stranger will answer my question.