My guess is a vole. Your first picture is my best clue.
My first encounter with one was a few years ago. It "burrowed" a trench along the dirt surface, but under the canopy of the grass (I cut mine long). The first thing I noticed was browning paths going through my healthy grass....they would lead randomly to my edging where I have bushes that offer cover. I had several paths from this animal leading out of my edged-in area where the bushes are. The first one I caught with a small live trap cage thing (I think I used peanut butter as bait) and relocated him in a field very far away. I caught a squirrel one night too...he was pretty mad at me. My next experience with one, he would not take the bait. So reluctantly, because I didn't want the poison affecting the food chain, I poisoned under the bushes where I found a hole I guessed he was living. Long story short, I came home from work one day for a different reason and decided to check on the burrows and poison. As luck would have it...they very vole himself was in one of the burrows and I could see him! He was not healthy and not doing well. I captured him and put him in the trap and put him in a place to die where to predator/scavenger could get to him. After work he was dead and I took care of him.