Same thing happened to me, had 2 hives wintering on a shared stand, one stronger than the other. A few weeks ago it seemed both had survived, bees on frames and flying. It seems what I saw was the bees from next-door robbing the smaller hive, which now is completely empty; no bees, stores or any sign if life. The one consolation is that the larger hive is now very strong with masses of stores, frames of honey, pollen, brood and loads of bees.
I don't think I had a mouse, it was just that the smaller hive couldn't defend against its bigger neighbour as I think the entrance was too large for them to guard effectively.
Lesson learnt,
Grant
