The MERL had a bat in its library:

First things first, we gave him rehydration fluid as he was very thirsty, having probably been trapped in the store for a few days at least. Fortunately, he was completely uninjured. We had a look round the outside of the building to try and see how he might have got in but with no windows, no doors to the outside, no louvres or obvious ventilation shafts we couldn’t spot anything. However, as a bat can get through a gap as small as a couple of centimetres and it’s a huge building, it can be almost impossible to find an access point if you don’t actually observe the bat going in or out.

A.