What is foil on doorknob when alone?

A foil on a doorknob when alone is a safety measure used to alert people to avoid entering a room or space. It is commonly used in public restrooms or changing rooms where someone may be using the space and does not want to be disturbed. The foil functions as a visual signal that someone is occupying the space and allows for privacy. When the person is finished using the space, they remove the foil from the doorknob and discard it. It is important to note that not all establishments use this method and individuals should check for other signs of occupancy before entering a room.