Oh Caz what a terrible thing and hope that Popy is OK
At least she stayed close and knows her home.
My first cat was a dumped cat who adopted me and I couldnt understand how she was in the house when I thought she was out and althoiugh she was full grown she was a small cat and she used to climb up and crawl through avery small gap where my small top bathroom window was open,,,,,it was no more than two inches!
The message is, keep all wimdows closed I am afraid.