My chickens free range most of the day. They have their coop to go into in case of danger. There really isn't any way for me to lock up the chicks and momma when I can't sit out there and watch them, if I do that then I lock the entire flock out of the coop. The chicks weren't alone, they had the entire flock to warn of the danger. I suspect that this particular chick just didn't listen to the others. Momma has been very diligent, but the chicks are becoming more active and tending to move further away from her. Since this is momma's first brood, I suspect that she learned something from this experience. For the most part, my flock has been pretty free of predators so I think they have become a little lax in their security. I needed to pick up one of the hens this afternoon and the rooster really went after me, which is unusual. He's usually all bluff but this time he was serious. I would guess that losing one of the chicks has him on high alert now.
More likely it was a large bird the did the deed. I lost a 10 week old silky pullet to very large black birds that visited one day while my flock was free ranging. It happens sometimes. Chicks are a target because they're small and easily snatched. Usually the mommas keep tiny ones close enough to protect them and be their cover, but sometimes it just happens.
So sorry for your loss.