Sorry for your losses. Unfortunately predators will show you your weakness in your chicken containment system. Most of us have suffered this at one time or another. In our area, we have several types of predators and in my opinion foxes are the worst. They are smart and sneaky. The chickens could range the grounds freely under the watchful eye of our dog. Nothing was ever taken on his watch. One day last summer I had a quick errand to run and popped him in the house and left. Fifteen minutes later I returned to find my mother in law in the yard with a broom. She had seen a fox. Unfortunately my beautiful large Jersey giant hen was gone. (Why she went out with a broom instead of sending the dog out is beyond me.)For this to happen so quickly, the only thing I could figure was the fox had been watching from a vantage point and when the dog went in he sprang into action.
A nearby neighbor had 17 chickens disappear in one night. They were inside the coop high on roosts, but the chicken door was a flap that was not secured that night. The foxes (I suspect more than one.) had carried the chickens away. All that remained was a blizzard of feathers.
What makes foxes the worst of predators in our location is that they will hunt and be out both day and night so just locking the chickens up at night isn't enough. Foxes will also carry extra food (chickens) away and cache them for when their food supply is lower.
