Yup, seen the same thing before. One fox killing all but one chicken during the night. Same conditions as you describe, but no door was open and instead the fox dug a hole under the fence.
Wow, that's a clever way to deal with fences! Wouldn't have expected they'd do that.
Generally, fences should be extended 8-12 inches into the ground to prevent this. Some animals can also climb fences so a completely secure installation has wire or boarding over the top too.