Sometimes they like to find a place, like a nesting box, to make a clutch of eggs. Muscovy hens will usually lay every egg of their clutch in the same spot, other mallard derived breeds will sometimes drop out eggs randomly in the run or out in the grass while free ranging. I have gotten my rouens to use a dog crate as their nesting box and I would say 90% of the time they lay in there but I still do find eggs out in the grass, in puddles, or in the run sometimes. I have seen them pop out an egg while walking and not even look back at it, it's almost like they don't even notice when they lay one, they make chicken hens look like drama queens.