There is a possibility they chose not to come back, theres a possibility they chose to leave for the days and only come home at night and theres the rare possibility they don't want to leave at all. Usually they know to return to where they are fed. However free range will always result in (over time) a few losses. At least one of your birds won't make it home one day and you'll have to be ready to face some losses. However, some people say they have the happiest ducks in the world and they'd rather dir free ranging than to live in a coop. It a topic similar to breast feeding or bottle feeding a baby. It's all personal choice and what works. There is no right or wrong. What you feel is best for your birds is the right choice, however remember you do have predators nearby so it is asking for some kind of invitation for trouble at SOME point in time. Losses could be very, very minimal and maybe only happen once ever, or they could be huge and right away. Do you have neighbors that free range? That's always a good way to judge the safety level of your area.