Middle Stall

There are three stalls in the bathroom.
After careful observation, I’ve noticed that whenever Stanley uses the left one, he comes out the right one. And whenever he uses the right one, he comes out the left one.
Stanley can’t explain it. It”s just something that happens.
So, I asked him what happens when he uses the middle one.
“I don’t know,” he said. “I’ve never used it.”
He stepped into the center stall and closed the door.
A minute went by before I knocked.
“Are you in there, Stanley?” I asked.
He wasn’t.
If you see him, call me?