Empty breech indicators for underlevers

Sounds like they are looking for the wrong thing. The potential energy in a spring gun is not stored in the pellet. They should check to see that there is tension on the underlever. Leaving an underlever or break barrel in the open relaxed position should be evidence. 

May be hard to describe but if you crack a break barrel or underlever open it will only go a small amount, if it falls to 90 degrees that would show the gun is cocked and could be dangerous with or without a pellet.
 
The Air Rifle/Pistol types that shoot at the range just north of Mesa, AZ made these little gizmos that should work for any action.

The parts are available at any craft’s store.

1/8th inch stick (wood) and a wooden ball (about 2” in diameter) glued on the end.

I suppose you could use any ball laying around.

Just stick it on the stick.

The ball is painted a bright orange/yellow/green, whatever.

I don’t shoot there but I visited the range a couple of years ago when I visited the grand-baby:



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