The younger, friendlier, confident cat (Thomas, the ginger and white one in a couple of the photos) worked out how to use the PetWalk cat flap on his first day in the house (albeit with a bit of encouragement and demonstration from me).
The older, deaf, timid one (Bob, plain black and camera shy) was blissfully unaware of of the presence of a portal to the outside world and has been using a litter tray in the Utility Room (where the cat flap is). When I’ve tried opening the cat flap for him, using the remote control, he’s run away.
I don’t know whether he saw the other cat using it or worked it out for himself, but yesterday I spotted him walk up to it, wait for it to open and hop outside as if he’d been doing it all his life.