Here in MN, our weasels are a little smaller than a ferret, I think. They can get in a very small whole, as previously mentioned. Since there didn't appear to be a mark on them, I would bet that that's what you had. Weasels will just make a bite mark (in the case of the flock of mine that was wiped out by one, anyway) on the neck, or under the wing and drink the blood. Since it happened at night, I don't believe that a rooster would have made a difference. Chickens are pretty helpless at night. Don't beat yourself up. You did your best. I don't think there is a 100% predator proof coop. Close, but if something is determined enough, it will get in. Enjoy the chickens you still have.