Check the cage really well, there may be a loose section that can be moved to form a gap. There is something you may be over looking. I doubt it was a snake. Snakes swallow their prey hole. If the chicken was too big to fit through the fence grid then the snake after feeding wouldn't be able to get back out for quite a while. It basically would have been trapped in the cage.
No feathers, no way out and simply gone.
1) Caretaker was distracted and didn't notice the chicken follow them out of the coop.
2) Human stole chicken
3)There is a gap your not seeing. One that isn't obvious somehow.
4)People from the future teleported the chicken in order to recreate the extinct breed
These are the only possibilities that I can come up with for a missing bird with zero signs of a struggle.
Riki
Distracted happened to my daughter. She had her school books, and was talking on her cell phone as she left for school. Walked out of the house locked the door and left. She never noticed that our miniature poodle followed her out of the house. The poor dog was stuck on the porch till I got home from work in the evening.