looks large for mouse droppings, rats perhaps?

Traps and peanut butter would be a good start, poison if you don't have pets.

1. traps and peanut butter inside for the current invaders.
2. bait stations around the perimeter of the building
3. plug any potential access like the opening in your pictures with this

https://www.google.com/search?...ceid=chrome&ie=UTF-8

Rats? Mice? Just be grateful you don't have a coypu problem.

Mice turds are pretty uniform grain of rice sized. Rat turds are larger and vary in shape, sometimes round. Looks like mice, rats are more destructive.

I use steel wool with PL400 for filling holes.

Do bait stations inside and out.

The pictures are mouse droppings which is little easier to get rid of then rats. Set some snap traps along runways, usually the along the baseboards, use bait inside and out. Be prepared with rodenticide that the mice may die in an inaccessible area like inside a wall. If you find any openings stuff them with steel wool or copper stuffit. If you shoot me an email with an address I can send you some good traps, bait stations and stuffit.

Yup, mouseturds. Use a nut (shelled) as bait, and smear a little peanut butter under it for odor. WIRE it on to the trap trigger. Mine in the shop learned to lick the p butter off, and never trip the trap. Also, the Chinese traps at Lowes have stronger springs and touchier triggers than the real American made Victors they are copied from. I suppose the Victors were lawyer-ized...