Foreign object debris walks are quite common and are often performed daily. But to spite these efforts, objects can still make thier way onto the tarmac because high winds can carry light objects over hundreds of miles from other areas. Also people can still lose items after the walk has been performed!
Imagine if they had warning signs – how much more effective there efforts would be. These signs would help prevent lost objects in the first place. Because its a proven fact that warning signs keep danger in mind!