It's not uncommon. University buildings are pretty open even today. I worked with a homeless guy who was a university employee, so he had a legit staff ID card. He just didn't want to spend money on rent. He knew where the empty rooms were that nobody ever used, where the showers were.
I bet every major university has a few people living/sheltering in campus buildings.
My somewhat anecdotal observation is that a lot of access has been tightened down. Some happened during COVID and was just never removed. It definitely varies. The campus I'm most familiar with, you could wander pretty freely and that's less the case today.
RMS famously lived out of his office at MIT for a while.