It sounds as if you might have had rats/mice there at some time as well – they are more likely to have shredded the bag. So it may have been rat droppings that you saw, as you initially thought. Hogs don’t tend to poo in their nest area, in my experience.
It shouldn’t do any harm to put some water in the garage for the hog, although maybe not strictly necessary if there is already some just outside..
Good luck to you and the hog.