Luckliy my son hasn't done that yet. When I was in the Navy, a freind from my squadron lived in the same rat-hole apartment complex as me. It was one of those 2 story box designs, with the doors facing the inside, with a courtyard and a pool.
This one, actually:
Sunnyvale, CA - Google Maps
We both lived on the first floor, and one Saturday morning, I am kicking back watching TV, and some movement caught my eye. I look over, and across the courtyard, on the second floor is his 2 year old son, running along the balcony.
I go grab him, and get him home, to find his front door open, dad is at work, and his mom asleep. She had laid down while he was napping, and he had woken up first. She locked and chained the door, but the little Houdini he had made an impromptu scaffolding out of a chair and his daiper box to climb up and take the chain off and open the deadbolt.
Luckily he went up, because he could have easily went the other way and ended up in traffic.