What information does Dr. Matthews share with the Master about the gate on the college's south-west wall?

Dr. Matthews tells the Master that the gate on the southwest wall is the only other entrance besides the main front gate. Yet, it seems like it was built to keep people out. There is no inscription on the gate, and it is not part of the college's expense records. The gate has been found standing open only twice. No one knows who opened it or closed it.