Ok, so I'm back.
The past few times we've entered the apartment, Moon has been walking out the door again. Both times his mom has been home, so we've ruled out he's missing his mom, I really do not know why he would miss her, he probably has a better relationship with his cat feeder than his mom.
I have to go with the possibility that he's trying to claim the area outside the door in the stairwell? However, he when he does walk out, he immediately regrets his decision and goes back in the apt, but only to do a 180 and look back out into the stairwell. Yesterday, my gf almost crushed him in between the door bc she saw Moon come in, but did not notice him poke his head back out the door immediately after he came back in the apt.
I have been using my foot to block his path out the door when I can, but he's a cat and has super fast reflexes. In the end, if he doesn't slowly walk back into the apt, I go over and pick him up. Still very scared of the day he realizes he has faster reflexes and can run faster than me.
Still not sure what's going on with Moon.