I always learned to put my mask around my neck when not in use because on the forehead is there perfect place for it to slip off and be donated to the ocean. Signs of distress often present as divers not following standard procedures that calm thinking divers would. Therefore it has always been my assumption that the distress sign was something that started out as "look for signs of a diver not acting right such as a mask on the forehead" and eventually someone who learned that starting saying "mask's on the forehead is a sign of distress" which is close but easily misinterpreted...