When I say square feet, I am asking how many square feet of pasture is needed per duck to keep them happy and alive. I have six ducks if that helps though. As for the previous owner, I know them very well and witnessed this with my own eyes. Their son (12 years old) decided to buy six ducks to get some extra money from eggs. He soon lost interest and the ducks escaped their enclosure (which was a free range enclosure I might add). After several weeks and/or months of them being escaped with nobody feeding them, they gained a lot of weight just grazing around the fields. I wish I had a picture but sadly I don't. When I asked them if I could take the escaped ducks off their hands, they were pleased to do so. This story is no hoax. I promise you.
There is a slight possibility that they found some source of food (maybe a broken grain silo) but this is extremely unlikely. Their farm is very well maintained.