I think that a water change is all that you can do, not that I have woken up finding dead mice floating in my tank. If it was only there for a couple of hours then I don't think that it would have been very decomposed, so maybe less body bits in the water?? I don't know, it sounded better in my head.
Reminds me of when I had a wolf fish and a mouse must have fallen in at some point because in the morning the mouse was sticking out of the wolf's mouth.