The exact same thing happened to me with Milo! When he and Bella were just 8 weeks old,weŽd keep them in a pen while we were out to prevent them getting hurt,but everytime we came home,heŽd be outside it!
I had no idea what was going on..it was really high (pushed up against the couch) and there was no door,so we were baffled for days!
Then one day I decided to put him in there,leave the room and peep through the crack in the door to see how he did it
I was amazed to see him CLIMB up the side that was pushed up against the couch,land safely onto the couch and then dive to the floor (my couch is quite low).Little monkey!!!