I used to own a hound dog. 12 miles in a day is NOT out of the bounds of what a dog would consider a reasonable jaunt.
Dogs also have a good sense of time. Dog probably knew when its humans were coming home, and always came home before them.
I had a neighbor who had that problem. People told him “Your dog is getting out” and he told them “it isn’t my dog” until finally we said “Yes, he yelps when the shock collar goes off, then runs around the neighborhood all day, then yelps when he gets back in his yard shortly before you get home”.