Signal Mirrors A Signal Mirror is the single most valuable daytime means of visual signaling. Even on overcast days, this signal mirror can be used effectively to obtain rescue. You can't be rescued if you can't be found. There is a reason that every military survival kit includes a signal mirror. Survival experts worldwide agree that no person should ever venture into the woods, travel off-road, get on board their boat or plane, or engage in any other outdoor activity without a signal mirror and a whistle.