It was an occasional occurrence at the biological suppliers where I once worked. It was based in an old Victorian villa & there'd a few escapes of rats which had bred. Hollow walls, hollow floors - plenty of spaces for them.
There was a mitigating factor. One of the managers, in charge of preparing microscope specimens for schools, stayed behind one night with an air-gun and shot one. Realising that it had a good chance of being infected with trypanosomes which would give him a batch of slides he went off to get a syringe. When he came back the body had been dragged away, presumably to be eaten by the other rats.