Well, she isn’t tagged, nor is she pregnant. I also assume that she doesn’t have anything obviously wrong with her, otherwise the vet nurse would probably have mentioned it (even though we didn’t have a full MOT).
So, step two is to stick posters around the local area and wait. If that doesn’t work, we might have to look at a) adopting her, b) naming her, and c) working out where we can put a cat flap (it’s not looking good at the moment).