Chickens have many predators. Here there are rats, cats, dogs, hawks, owls, eagles, weasels, mink, fisher, opossum, skunk, raccoon, fox, coyote, bobcat, bear, and I have a hard time believing you have none of those.
You want to free range. That is fine as long as YOUR birds remain on YOUR...
Your birds need to stay on your property. It really is that simple.
As for your missing chick, a 6 week old is easy pickings for a lot of predators. You have no rats, cats or hawks?