I believe that cats have a way of finding ways of opening window screens. Just after I moved into this house, I remember driving in from the grocery store, and saw Tiger out and about searching out her new area. I thought to myself, " I do not remember letting her outside". Sure enough, they had found the hook that keeps the window screen attached, was loose, and they unhooked it, and not only Tiger got out but a few of the others as well.

..............wayne