I'm so sorry for your losses.The one dog that has access to that area (Odin) is 100% trustworthy, and as mentioned he was not in there when the ducklings got stolen or when Dorothy was killed.
My chihuahuas, rat terrier and lab, who are also trustworthy, can't get in there because it's completely fenced with a 6 ft fence on 3sides and an 8ft fence on the other to keep the goats in. There are also no holes dug around, we looked. Which is iI think dogs and coyotes are out of the question.
We do have those black garden snakes, but I can't imagine one of them stealing a muscovy. ..but I do wonder if maybe what the lady saw wasn't a run away pet or something
I'm in deep south texas