Well, that was unpleasant. Let the ducks out, fixed up the fence as best I could, did a walkaround the enclosure. Watched the ducks for a bit and then noticed the distinct absence of a huge white duck. Checked the coops, checked around, checked again. Couldn't find her.
And then I spotted something white between the fence and the coop.
There go the plans on breeding Aylesbury. I have no idea how she got there, she was the size of a scovie drake! Got herself wedge and couldn't get out.

Probably suffocated, poor thing.