Yep, if you don't make it predator proof, you WILL lose chickens.
Common night predators are foxes, raccoons, weasel family. But don't count on the "night" part, coons and foxes can be out during the day as well. I had a fox come into my fenced area (better fenced now!!!) just as I was coming out of the barn about 3 PM in the summer. If I hadn't been coming out just then I would have lost a hen. Those foxes are FAST!
And I found a coon going into the coop in the barn about the same time of day a different time. That one got a free one way trip to the woods, it had already gotten into the barn (I think through a woodchuck tunnel) the night before and killed a hen it tried to kill the night before. That hen had been missing for a week or more, turned out she was broody (non fertile eggs) outside of the fenced area back when we let them range. Heard her screaming at 3 AM, ran out, coon was up the tree. I stupidly put the hen in a not quite predator proof stall since I didn't want her to go do a broody sleep in the nest boxes in the coop. BIG MISTAKE.