Which door was opened? The one on the outside that swings, or the one by the ramp that slides? Is it enclosed? There are things that can slide those door, but if hers is anything like mine, the track gets full of poop and isn't always easy to clean, making sliding much more difficult.
I'm with
@Shadrach somebody forgot to close it and made up a little story.
I definitely think there would be feathers inside also. Those are very small for 8 full size chickens. I use one as a grow-out coop for babies only.
I do hope she finds some that ran away to hide, and may turn up.