It really is hard. And we just do the best we can and sometimes that isn't enough. I never even knew we had snappers here in our cold mountain river until we had flooding and one washed up on the road. I helped get him back to the river boy did he stink and he was huge. Growing up in Florida I...
Yes they seem to know things like that. Especially if one was already attacked. Another member on here just lost one of her ducks to a snapper. They are gruesome predators.
Only thing I can add is if there is one you’ve seen there are probably more. Set out traps you tube shows how. After seeing what a snapper can do I sure hope your ducks don’t go back in till you know for sure it’s clear.
https://www.backyardchickens.com/articles/the-chicken-worlds-worst-chore-culling-the-injured-and-sick-babies.72140/. I’ve never used this on any of my ducks. But have on my chickens when needing humanely cared for. So sorry :hugs but are you sure she won’t heal with out surgery?