This lock sign (left) is often used for staff members at the airport to access high security areas which are non-accessible to the public. Staff members need to swipe their identity badge in front of the key sign to enter.
This sign is occasionally found on beaches. Weston-Super-Mare is famous for having this sign and for its treacherous sinking mud flats. Each year at least one or two victims are claimed by the mud. I think anyone who ignores this sign must be crazy!!!