I hate to be a Debbie Downer, but now that whatever it was found a meal, it may be back for dessert.
We have had to do a lot to prevent losses here, mostly due to coons, rats and snakes. We put scrap cage wire between our shed and the attached coop/run, we put down some concrete over that, we put up a solar electric fence, and we are in the process of adding a two foot high ring of stones around the coop. I would look around for whatever I could find and reinforce the coop every way from Sunday.
Another thing we changed is how we introduce little ones to the flock. Once they are feathered, they go to a sectioned off section of the coop and stay there a couple weeks so everyone can get used to each other. Then they get added to the flock, usually at about 8-9 weeks. There is a bit of chasing, but nothing bad, and they mingle quickly. It seems to help them acclimate and it adds protection .
*fingers crossed for no more losses*