Those were some very special cats, esepcially considering that you had TWO that did it at the same time. I’ve had 16 indoor cats and about twice as many outdoor cats over the past 44 years and not a single one of the indoor cats has ever “decided” he wanted to become an outdoor cat. Cats are inherently lazy, interrupted bursts of intense activity, followed by plenty of sleep. They enjoy comfort and safety and a plentiful food supply. Being indoors allows them to have the lifestyle that any cat would desire and after a couple years indoors, they develop a fear of open places.
Now, an outdoor cat cannot see this because it has a strong distrust of enclosures and your home is an enclosure and their worries about an escape route overrides their natural laziness because he knows that he can still be lazy outside, but once he realizes that your house is a trap, he get’s very uncomfortable.
An indoor/outdoor cat may decide he’d rather live outdoors, but that decision is based on being an indoor/outdoor cat (i.e. he thinks he can get back in and take a safe nap whenever he wants it) and is usually influenced by some factors such as a new child, pet, or changes in “his” people’s behavior.
Yep, they were indoor / outdoor for about 5 years then they stopped coming inside. I just figured that cats have a mind of their own and they would do what they prefer..