This might be unlikely for your duck depending on your setup, but my Muscovy was attacked by an owl last month. The owl flew into their covered run through the open door (we hadn't locked them up for the night yet because it was middle of the afternoon). If my husband hadn't happened to see it, we may have never known it happened, assuming the Muscovy managed to get away. It had a hold of her on the back and she was beating at it with her wings.
She didn't have any puncture wounds and didn't lose enough feathers to be noticeable. She looked perfectly fine, except she didn't fly for weeks and she was irritable for awhile after the attack. She wouldn't come for treats, stayed off by herself, got annoyed when I approached her. Is there a chance your duck got hurt somehow and the injury just isn't visible? My Muscovy didn't act as if she were in pain, but the lack of flying (she loves flying and does it often) made me realize she was likely hurt worse than anything I could see.
