Story time! We recently bought a rural property with a burnt down house on it and are going up every few weekends to clean it up. I'm up there this morning picking up random sheet metal and yeeting it into the trash pile. Suddenly I hear frantic squeaks....I look, and I accidentally yeeted the cover of a squirrel nest (I think squirrel)! I got the baby, got it out of the baking sun and made a new nest under another piece of cover. Still, holding a baby squirrel so new it's eyes weren't open was pretty magical! The kids (I have a 7 year old girl and a 9 year old boy) got to each hold it for a second too! Ideally we wouldn't have touched it, but the existing cover was mega-gone and it had to be relocated to safety out of the sun, so we got some brush and put it under a smaller wood plank right near where we found it, hopefully it's mom finds it, no one will be up there for the next few weeks so it won't be disturbed by people again.
If it was on the ground I would assume bun nest. I'm not an expert on either critter though so hopefully someone else can chime in.
It had a long worm-like tail. If it was smaller I would have thought mouse, but the leading theories are squirrel or mole.
My previous experience in remote trash piles says pack rats, lol.