Getting a Taste of the Wild

Alex has always loved the outdoors, but unfortunately for him (well, fortunately for his lifespan, unfortunately for his desires) he's an indoor cat. He used to be responsible.  Several years ago we even trained him to sit nicely and not run away.   But that time is gone, and he is now irresponsible.  In fact, he takes every opportunity he can to bolt out of our house if someone leaves the door open for even a split second.  He used to be super eager to get outside but then was too terrified to go anywhere, so he'd just cower on the deck.  Or if he gets out the front door he immediately seeks shelter under a car in our driveway.  But each time, he gets a little bolder.  Now when he gets out in the back, he immediately goes under the deck, which is really annoying, because we can't get under there, so I have to use a long pole or something to shoo him out of there.  He's now even to the point of running out from under the deck and trying to scale the fence to get out of the back yard.  It really is concerning, because one time he really did get lost for a few days.  So we try our best to keep him in the house, and if he gets out, it's my job (if I'm home) to corral him immediately.  Today when a kid reported he had escaped, I went outside to track him down, but instead of hiding under the deck, he was casually, but with a degree of tentativeness, perusing our shrubbery.  He saw me and paused, so I watched him for a few seconds until he went back to his exploration, at which point I nabbed him and threw him back inside.  Another crisis averted, but it worries me every time he gets out.