Featured Stories on BackYard Chickens
Are there predators in your area? I had two ducks go missing on two separate occasions and later we figured both had died because they never came back. The problem was all fixed by building a run for our birds to make sure they stayed safe through the night.
Maybe they found someone else in your neighborhood that provided food or water, like cats. I haven't heard about ducks going off to lay eggs, other people might though.