Underneath the stairs would be a nice warm place to sleep for an opossum. They do not hibernate. If you can't see it breathing, sprinkle some flour around the stair around dusk. When the opossum goes out to find food, it will leave tracks in the flour. (If there are no tracks after several days, my guess is that the smell would confirm that the animal is not alive). Or you could take a broom and gently nudge the animal. You can find pictures of their tracks at:
http://web.extension.illinois.edu/wildlife/identify_tracks.cfm
If you need info. on trapping the animal (or what to do with it if it is not just sleeping), please let me know.
