Currently on holiday and have left the cats a home with a cat sitter coming in twice a day. We got a new door with a cat flap shortly before we left (never had a flap before and last year the cats were kept in for 2 weeks when we were away). When we left they were managing to get out the flap but weren't using it to get back in. The cat sitter seems to think they are using it on occasion but we also left a key with the neighbour who has also been letting them in. From what they have said the cats haven't always been in at night whereas the had never been out overnight before we left.
The neighbour even carried one round from the back door to let him in the front door! We have now recruited our neighbour as new cat slaves although not sure their dog will be impressed. She said he looked a bit rough and could do with brushing but didn't think he'd tolerate her doing it. Scared of what we will be going back to tbh. Should just be glad they still seem to be at home and haven't run away because we aren't there.