Since she was not free ranged just like my Duck was not it was probably not killed by something it should not have been able to get into. I hate it when we lose one and don't know why.
So sorry for your loss. I had a duck years ago that was in perfect health, not a mark on his body and young yet laid dead in the pen. The only thing that I think might have killed him is if the other drake I had, his brother, grabbed him by the neck and killed him. Still not a mark on him...