It does sound more like a predator and possibly a hawk since it would only take down one duck and could have gotten scared off before it had a chance to eat much. We have a Predator and prey thread here why not go there and see if anyone can pin it down for you, but be wary what ever did this...
@gibmom
Have you noticed any of the drakes ganging on on another? Years ago I had a Muscovy drake kill another Muscovy drake but I have had several since and even though they fight non have killed another since that one time. Could have been an illness also they can hide sickness so well so...