i get you guys but i'm wondering - isn't this meant to be for use only when you would have the gun loaded? I don't think the suggestion is totally rely on it when safety is at stake - "oh it's on green so i'll chuck my gun in the boot, go home and think no more"
but, you're in your hide and waiting, having the gun loaded is ok, but you took a shot and the pigeon landed belly up so you retrieved it, got bored, ate your mars bar, then you hear a flutter.... it would be handy to be reminded if you have one up the spout or not? Probably not the best time to discharge the rifle just to check as