To repeat: foxes will come out in broad daylight and snatch a chicken and leave no clues or perhaps a small handful of feathers. I know because I saw him out the window and he and my grown roo were gone before I could get to the door. The first time I saw a fox get one he was plucking the hen in the yard and I was able to save that one but they have given me a lot of problem since and that is the only time they have left significant sign. Foxes are stealthy. I would suggest an electric fence. We have buzzards and I have never known them to kill a chicken. Hawks will do a swoop and snatch especially on smaller birds but ususally I see a hawk in the area casing the set up before he acts. Dogs, cats, racoons will all kill in daylight but usually the body does not disappear. However, if chickens are frightened by one predator they may go into hiding and either return safely later or be caught later by that predator or a different one.
Sorry for you losses, it is perhaps the worst part of having chickens.