Not that this helps, but, as a rookie chicken-raiser, I had raised 55 mutt bantams a few years ago and refused to turn them out. (We live in the country, and our property is not fenced in.) After much contemplation, my MIL convinced me to turn the chickens out during the day and to put them up at night. Her reasoning was that they'd get to hunt for insects, scratching around--they'd be happier. Since I had other chickens that I STILL refused to release--raising them for meat and eggs--I thought I'd could "spare" the bantams. By the end of one week, we had 2 bantams left, then they disappeared. (Before you ask why I waited so long, I had decided to get out of the "bantam business" at the time--we wanted dual purpose birds--so having them eat insects in my flowerbeds was a bonus. If they raised while free-ranging, that would've been GREAT!) It got to the point that I started getting predators INSIDE my other pens during the day after I had lost all of the bantams. After trapping 1 stray dog, 3 possums, and finding coyote scat, we finally put a roof on all of our pens. That's the only way we keep our birds alive.
I envy you all for being able to free range your chickens. It's just not possible here.
Hope you find out who the culprit is. Best of luck to you.
Chanda