I kinda assume that maybe an owl was smart enough to get through the one hole... And was taking one a night???? And maybe since they are just half grown the stupid idiot chickens didn't notice so didn't fuss?
I think an eagle, attacking during the day, the other poultry would have seen and made a ruckus. Also the eagles often yell/call after a kill, since they are so proud of themselves.
A silent night attacker makes the most sense. And the grow outs are still small enough for most anything to carry them off. (No blood, no feathers)
I have sat in different spots around the property, nice and quiet for 10 to 20 minutes at each spot...and no grow-outs

lots of little fledgling golden crown sparrows, fox sparrows, etc....but no chickens.
The 6 survivors somehow managed to get from the veggie garden to the duck yard... Not sure how... But I have lost no ducks, and their yard is also very overgrown...but the ducks come when you call, so I KNOW I have all of them.
The ducks also have ducklings that nothing has eaten.....
It blows my mind that other people free range... HOW THE BLEEBING H DO THEY DO THAT?????
My grow-outs, even if they managed to get from the veggie pen into the duck pen, were still PENNED, and had lots of cover.