It's hard to believe a predator was the culprit with your other 2 ducks not injured or acting terrified after the ordeal. Do you have any neighbors that might have taken him or live where someone passing by might have seen them and helped themselves. Reason asking we have had members who ducks were stolen. Unless he just had the opportunity to fly and took it. But if he had females seems he'd have come back.
Very sorry he hasn't returned.
Very sorry he hasn't returned.