Lets see... battery failure no bang!
Silly. Solar powered. Or wind powered. Just stick the watch “someplace” where some “air flow” might occur and charge it up. So now the bad guy needs to steal the watch and the gun? Simple. Keep the watch locked away inside a transmitter proof cage. You can leave the gun just lyin’ around. Which most guns do anyway. Lazy pieces of crap.
I’m trying to remember the name of the gun writer/hunter who said the most terrifying sound in the world is when your firearm goes “click” instead of “BANG”. Especially when the animal you are aiming at is higher than you on the food chain.
I think it was Peter Hathaway Capstick but I’m not sure.
Put a crank on it to charge battery, or a solar cell.