Rats, Foxes and Bats
I used to work for the NHS in Notts. Two massive hospitals over two sites.
The IT building at one of the trusts had a family of foxes living underneath it. We found this out because we could hear the cubs crying from the server room.
A few years later the same building had an outbreak of rats underneath. Property Services sent their "all life is futile" guy down who set up traps and cameras, the next day shared the YouTube links where they'd been caught and killed.
Across the ring road there's two comms rooms. When a massive area of the grounds was being leveled for a car park the building with a lot of legacy kit in wasn't allowed to come down because a family of bats lived in there.
This was all some time ago, I dare say beavers have moved into the first campus, the second probably has Syrian refugees hiding out.