Device to show if a door is locked

Situation: You sit in an office. The nearest toilet is far away. You get up, walk the distance and when arriving - you find that someone is already in there. Either you stand outside the door listening to whatever they are doing in there, like some pervert - or you walk all the way back. Only to repeat ten minutes later.

Situation: You are sitting in your living room and suddenly you worry if the front door is locked or not. Either you keep on worrying  or you get up and check the door. Again.

Solution: Some kind of device that is easily attached to a door that senses if the door is locked - and - display that status with a easy-to-read-from-a-distance sign. Kind of like they do on planes.