On the left side of the door is 3/4 of a green square and then another one right opposite of it on the fuselage. My best guess is that the squares are to align the door, but any help would be appreciated.
I don't think it is used to align the door. Though never having worked a -400 I'm pretty sure from pictures I've seen that it is the same door as on the classics. The door on a classic doesn't need to be aligned. It is pretty much lined up or it will not lock.
Now what are the markings? From the location, it could be the hook-up location for a lanyard that keeps the door from swinging all the way open on the ground. We have them on some of our aircraft. It just keeps the door from swinging all the way open. This lanyard is not installed all the time. It is there for ground ops only and by flight crew only under very specific conditions. Of course it could not be used in flight due to pressurization.
It's a french plane.
Probably some anal compulsion to know where the hinges are located when they're out of sight.
Actually after looking in Chapter 11 they're not listed so my personal best guess is alignment marks for the door panel installation or maybe my first guess was right and they're "cut here" marks for an emergency?
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
Just realized, that door opens forward, so its not what I thought. It is in the right place for the upper hinge, but why do you need to know that from the inside? Its not a cut here mark, AFR is taught to avoid doors when cutting. Too much structure and the possibility of a slide or door assist blowing.
It looks like a designated area for placing a tamper-proof security seal sticker. These are placed on the doors when the aircraft is parked overnight in a remote area. If the stickers have to be placed inside the aircraft, then would their not always be a door that would not have a seal?