I don't quite understand, either. As I say, she was out Sunday and it would have been quite normal for her to come back in the early hours of Monday. Ideally, I'd like them out less time, but it's just the way they are. They're young and have stuff to do!
My hope was that the chip company would contact me as well as the vet, but clearly neither had. When I got to the vets, they showed me my details on a piece of paper and they were all correct - both address and telephone number. The chip company told me that she'd been reported missing by them, but clearly had got no further. Given that she was probably taking up valuable space, etc. I'm surprised they didn't try harder to contact me.
I am embarrassed about how polite I was about it all, but I guess it's done now. She's home and I'll know to check with the chip company earlier now.