Let's imagine you're using the bathroom. I know, it's a far-fetched scenario, but follow me here. There you are, sitting on the rim of porcelain, unaware that at this same exact time, another person has the urge to go too. Since you're a decent person and have closed the door, the other guy knocks before he will enter. How do you respond when you hear a knock while you're on the can?

"Someone's in here."

Why do we use the codeword someone? Are we trying to keep secret our activities in the restroom? The guy knows you're not some messenger who hangs out in the bathroom; you're not the user's secretary. He knows you are using the bathroom. As far as anonimity goes, admitting you are the user gives him no more clue to your identity. Saying "I'm in here," provides the same help to the man on the other side while acknowledging you have nothing to hide by using the bathroom.

Someone doesn't see the big deal.