MISSING DUCK NO SIGNS OF STRUGGLE

[backyardbirds394]

The part with the tree is also covered with net btw. They are 6 months old from birth, and all of them can fly. This one has always been more extroverted than the others i.e. quacking, pecking, canonballing, etc.

 

[Soon2BChixMom]

Is it tightly covered with the net or are there openings?
The lid propped up on the left hand lower side of your pic looks as though something could slip out or in?
You really can't blame a neighbor or a person unless you've seen them in your backyard mulling around for no reason. Ducks quack (well most ducks not muscovy) - and generally would raise a ruckus if something was carrying it off.
You might never know what happened. But, take a real good look around your set up and see if there are any spaces a duck or other animal could squeeze through.
This looks like a hodge podge of materials compiled together which could leave a space that we might not think about as an opening, but there just might be enough space somewhere.
Chicken wire isn't really the best wire for predator protection either. So, look around to see if there is any ripped that would create a hole.