Ice fishing Tip up lights
I also have reflective tape on my flag shafts so I can tell by a quick flashlight scan if a flag is up. That helps against encroachers in higher traffic areas. A flashing red light sometimes draws them in.
I purchased some of these a few years ago and they still have the same batteries in them. They work awesome! I keep them in my bibs pocket all season. No switches, no wires, just clip them on the flag shaft and wait. When the flag goes up a BB inside drops and it lights up. I have never had one fail. Simple, small, lightweight. when you pick up the tip ups at the end of the nite, just unclip and put in your pocket til next time. The best I've used