Unless that girl's head is blocking the view of a card swipe, I'm not seeing one anywhere on the walls by the door.
http://www.courant.com/community/new-haven/hc-annie-le-yale-missing-photo,0,2661037.photo
I work for the University of California in Berkeley, and although it is probably different at Yale, I thought I might be able to shed a little light on this subject.
Most of the common buildings, buildings where classes are held are unlocked all day or until 6pm or 7pm whenever the last classes take place. After that, you need a key card to enter the buildings. As for the labs here on my campus, common labs are sometimes unlocked during the day, but certain labs require key card access or you have to knock on the door to have someone let you in.
Hope that shed a little light, I wish we could learn the rules or practices of Yale though.