What's that scratching noise inside the walls?

My spouse thought she heard a rat in the attic space last week. Well, it's happened twice before so I went up to investigate. I didn't find anything. I did however spray some polyurethane foam in any cracks and holes that I thought a rat could get through from the walls below. Then I went under the house to do the same. Again, I didn't find any rat sign. I did see some fresh opossum poop. I brought a garbage bag with me to collect the remains of the cat and small dog skeletons this time. I've been meaning to do this since we bought the house. Since I had the bag, I threw the poop in too.

Back to the main task. Plugging holes. There's a big hole where the drain for the bathtub comes through the floor. My spouse and I have suspected that opossum have crawled up through this hole in the past and hung out in the space between the tub and the wall. Since the hole is too big to just spray polyurethane foam into, I stapled Tyvek over the hole. With the job done, the family settled in for a night of rest.

Didn't get any. At about 11PM we heard a bunch of movement. I knew immediately what was happening. I'd inadvertently trapped an opossum in the wall! Crap! The poor animal moved around ever twenty minutes or so. After about 2 hours it started trying to scratch its way out through the drywall (that's what it sounded like at least). I was seriously afraid of having to deal with a rogue opossum in the house.

So at 2AM I put on my "under-the-house" clothes from the previous afternoon and all the necessary gear and went under the house to free the damned rodent. I also took some grapes to lead it to freedom. While I was under the house I did not see or even hear the opossum. Thank god. I left the crawlspace hatch open as I left.

The next morning I noticed that a vent screen was pushed aside. This was the escape route that the opossum used ( I guess it didn't trust the freedom trail of grapes). I suspected that the opossum had vacated the area, but I still rented a live trap and set it just in case.

It's been a couple of days now and I haven't caught anything in the trap. I went under the house tonight and resealed the hole under the tub. I really don't want to have to go under the house again to free the damned opossum. If it's still in the wall I'm likely to just cut a hole in the wall, reach through and grab it. I'm not sure what I'll do once I grab it though. Maybe I'll devour it since I'll be so drunk with sleep-deprived rage.

UPDATE: No scratching noises last night. No opossum. No late night snack for me.