A man phones a mental hospital and asks the receptionist...
A man phones a mental hospital and asks the receptionist if there is anybody in room 27. She goes and checks, and comesback to the phone, telling him No, the room is empty. "Good," says the man. "That means I must have really escaped."

An invisible man is here to see you
Nurse: Doctor, there is an invisible man in your waiting room.

Doctor: Tell him I can't see him now. Next.