I have wondered about this very same thing in the past too.
Back in the days when you could accompany people to the gate to send them off, I waved to my parents as they boarded a Delta MD
-11 Atlanta-London. Being fascinated with airplanes, I stood at the gate window and watched all the flurry of activity as the ground crew prepared this enormous plane for takeoff.
Eventually the doors closed, the jetway started to creep back and it was obvious they were ready to go. Right then I noticed that there was a little door open (the same one you mention above). I started to get kind of concerned...did the ground crew notice? Had they left something amiss? I started to wonder if I should say something to someone. Then at that very moment when I was about to talk to the gate agent, one of the ground crew pointed at that little door and some guy got on an elevated contraption of some sort and closed it.