Took my first trip to Alcatraz Island a week ago, and what did I see but a very well-placed visual signal -- at least back in the day. If you can't tell from the photo, that's a shadow board. When the island served as a prison, this shadow board was located in the kitchen area. If one of the kitchen's knives was not in a cook's hand or hanging in its place on the shadow board, it was immediately apparent. Then locating that knife became paramount very quickly. A prison is not a place you want a knife on the loose.
P.S. Thanks to my sister, who took the picture at my request.