It
was residential accommodation in the early 1980s. I worked with a chap who used to bring a large rucksack into work every day, but never seemed to open it, before taking it out with him every night. Eventually, it was revealed that he was living on the top landing of the stairwell there. He went home to Worcester after work on a Friday, and came straight up from there on a Monday morning, so he only needed four nights here each week.
Long before the days of large-scale homelessness, the cleaners never locked the communal door to the stairwell when they left in the evening. He would sit on the wall of the SGI until he was sure it was clear, then go in and establish himself on the top landing, making sure he was gone before any of the office or shop occupants arrived in the morning.
He had shelter and some access to electricity and water/washing facilities - and they had an (unknown) extra security element.
He did it for about a year and never got caught.