Yep I think ducks can free range just like the chickens. Once they're aware that they have a safe place to sleep at night, I believe most ducks put themselves away. But they're not like chickens who become completely inactive at night, ducks can be nocturnal-ish... but they seem to prefer a safe place at night.
Your ducks should know now what their bedroom is, but you might need to train them with peas or something for a little while to get them to come back when the sun's setting (especially if you change the house). I have a smallish fenced-in duck yard for mine, so I don't know for sure if what I'm saying applies for ducks on a large acreage, but that's what I've heard.