I turned around and then heard little giggles that sounded like they were coming from the entry way...and they were! He had escaped.
And was very happy about it! :) He is pretty safe on the stairs and, obviously, knows how to get down. I am not ready to take the baby gate down yet, though. I like knowing where he is and that he can not get downstairs without me. The gate will stay up for a few more months, at least. He has not tried to escape yet today, so that's a good sign, right?!? :)