Its not squirrels, rats, gophers or rabbits. Hmmm... Groundhogs? Prairie Dogs? Weasels? Badgers? The droppings would suggest (to me) something that eats some meat. Don't ask me how I arrived there.
Might be time to invest in a Critter Cam. I have one. It is amazing what walks through my yard in the middle of the night. The only thing I've had in my greenhouse was a raccoon and he came in through a window.