Why is Gus at the mini mall and what does he say about Margo?

When Q and his friends reach the old mini mall, three men in masks show up. One of them is Gus, the guard at the SunTrust building Q and Margo visited. Gus and his friends are urban explorers who enter old buildings, take pictures, and leave. When Gus was a senior, Margo, then thirteen, tagged along, but she always liked to stay in the places that they went to. She often seemed depressed and wrote in a black notebook.