We have quail cages the mice squeeze through and let me tell you there is nothing like turning the lights on and having mice suddenly leaping in you to get out of the cage and hideOh wow! Sounds a bit like what happened to me a few months ago but it wasn't a possum.
I stuck my hand into a bag of hay so that I could grab some and give it to some mice. A FAT RAT WAS ON THE HAY THAT I PULLED OUT RIGHT BY MY HAND.