Since he's mostly indoors, Sebastian probably hasn't gone too far. My cat, Sofia, is also an indoor-only cat and she's gotten out twice. Once she followed my bf to the laundry room--at night too! She wasn't too far away, but obviously freaked and waiting for us to come get her (rather than coming to us when we called). I found her that same night, huddled next to a neighbor's apartment.
The second time, I was bringing in groceries and she ran out and just kept running! I think she thought we playing a game. She was running along happily (just fast enough so I couldn't catch her), UNTIL a car parked nearby, some people jogged along talking loudly. The oustide world was not welcoming and she panicked. Luckily, she decided to run to the downstairs apartment and I was able to corner her.
Now she doesn't even try to escape. She just looks warily at the door whenever we open it. I think her two mini-outdoor-adventures quelled her need for kitty freedom.