There are a number of predators that could have killed it. Start with turtles and work your way up to mammals (raccoon, fox, coyote, etc), including domestic ones.

Don't keep domestic ducks unless you can herd them into a barn or other safe enclosure at night.

Sorry: I meant to commiserate with you missing the little duckling! What I meant to say was securing them at night may prevent future losses.

Good duck luck!

Poor little guy. I hope you find him/her.

When they are little just about everything preys on them. My brother, who's an avid fisher, saw a duckling devoured by a large mouth bass.