A few days after my last post, I went to feed my chickens in the morning and found two tiny little ducklings sitting right in the middle of the flock. One had an injured leg and I couldn't find mama duck anywhere - I also found no signs of struggle (usually there will be feathers left if a predator gets a chicken or duck).
I decided I better catch the ducklings and put them in safe place. Boy did they give me a merry chase! It is amazing how fast a little duckling can run!
I did finally catch them, and now the little ones are growing up in my "nursery" swimming pool inside the chicken coop. The little one with the injured leg has healed with no sign of weakness. I'll put them out with the other ducks and chickens as soon as they grow too big to fit through the fencing. They seem to be a bit more "wild" than other baby ducklings and are very afraid of me. I think that they'll warm up once they get out with the other fowl and see how they react to me showing up with food.
Sadly, I still haven't found any sign of mama duck.