Definately sounds like a weasel, so you're going to want to close up the fencing into a much smaller mesh. They can squeeze their bodies like rats/mice to fit through smaller spaces. The top too, since they can climb pretty well.
Now that it has had success, it'll be persistant about getting in again. Most every predator after having a successful hunt will put more work into a future hunt if the access is changed from easy to hard. They KNOW there is tasty chicken in there, so you have to proof the coop to keep out a hungry AND knowledgable critter.
It's like giving a kid free access to the cookie jar, then hiding the cookie jar and putting a lock on it. How hard will the kid try to get the cookies before admitting defeat?
Now that it has had success, it'll be persistant about getting in again. Most every predator after having a successful hunt will put more work into a future hunt if the access is changed from easy to hard. They KNOW there is tasty chicken in there, so you have to proof the coop to keep out a hungry AND knowledgable critter.
It's like giving a kid free access to the cookie jar, then hiding the cookie jar and putting a lock on it. How hard will the kid try to get the cookies before admitting defeat?