I would place a bet that a predator got Aflac, or Aflac got away but is currently lost. Ducks don't usually wander off if they have a flock to be with. Either all go, or none go. However, if it was a predator, you need to keep your other ducks locked up in their coop for at least a couple days. Often once predators get one, they come back for the rest at some point. As far as determining what predator could have attacked, I would rule out a hawk because any kind of Peking or other large duck could not be taken away by them. The hawk would have to kill on sight, so search your property and the land nearby thoroughly. Other than that it's anyone's guess on what or if anything attacked. Best of luck on finding Aflac.