Apparently a mouse had got itself caught in a trap I had set out, but only by its arm. The mouse then ran around the side of the refrigerator (pooping several times apparently) and squeezed inside the refrigerator's vent cover. I initially tried to pull the mouse out of the slot it crawled into, but it emitted a pitiful squeak. I ended up taking the cover off and letting it go outside. It took off ok, so I'm sure I'll be seeing it in my kitchen again soon. Next time it may not be so lucky.
