A mouse got inside my city hive because I waited too long to put on mouse-proofing mesh.
From what I can tell, the mouse (or mice) was in the hive for a long time and scared the bees, queen and all, into a honey super that I had placed above the inner cover during a late fall feeding.
For an entire colony to abandon the brood nest by squeezing through an inner cover hole to cluster inside a honey super — that’s not normal.
I should have put my mouse-proofing mesh on as soon as I knew I wouldn’t need to move the bottom box for the rest of the year.
UPDATE: I use 6mm mesh to keep both shrews and mice out now.