It took him three weeks, but Sebastian finally discovered the cat walks (Lance figured them out within a week and a half).  I came out of the office, and saw Kate sitting on the walk leading into the hideaway (through the hole in the wall), pawing at someone on the other side.  I thought it was probably Maggie, but then, lo and behold, Sebastian popped out through the hole!  He had apparently been up there and in the hideaway for some time, but couldn't quite figure out how to get down.  He kept tentatively coming down the ramps, but then getting scared near the end, where he has to jump down to a crossover to get onto the lower ramps to the ground, and would turn tail and run back up on top of the kitchen cabinets.  It let me get some good pictures, and eventually I was able to encourage him to come all the way down.  He'll get the hang of it before long...