I don't have an exact depth, but I know they can dig down a lot, they are one of the true burrowing species. Mine have buried themselves several inches down before, I always put 6-12 inches of substrate in my indoor enclosures for that reason. They are also great climbers, when you do find him I would put caps on the top of that garden wall so it sticks into his cage 1-2 inches, it will prevent him from climbing out again (if that is where he is). As for not moving around, I don't think it is that they do not like being outside, I think it is more that tortoises usually don't like change, so I would guess that he is probably hiding in his enclosure and scouting it out when he feels it is safe and no one is around. My female that lives outside spends almost all of her time underground burrowing, I only ever see her on the weeks her chicory garden is full grown.
I also suggest putting up fliers and calling pet stores/animal control and letting them know hes missing, you would be surprised how many people will see a tort and decide to keep it or something like that, not even thinking about it being someone else's pet. Even if he is in his enclosure still it can't hurt and will help you if he actually did escape.
I also suggest putting up fliers and calling pet stores/animal control and letting them know hes missing, you would be surprised how many people will see a tort and decide to keep it or something like that, not even thinking about it being someone else's pet. Even if he is in his enclosure still it can't hurt and will help you if he actually did escape.