A friend of mine that has a 1/8 acre pond in SE Texas just west of us had the same incident happen. The culprit was a snapping turtle. They swam up from underneath and grabbed his ducks by the legs and pulled them down and drowned them first...then ate them. He had one duck that survived, minus 1 leg. You can catch the turtle with a baited hook at the surface. Good luck!

...when in doubt...set the hook...