poor puppy. chances are it probably gave up and realized it's stuck in the yard...all by himself.

the two runaways are in danger for many reasons. they could be hit by a car, poisoned, stolen, etc. i would next time you return them i would ask her how they are getting loose. if she knows why tell her she needs to fix the problem or you will call the police or animal control.

i would tell the owners of the rottie that he can stick his head out. they may not even know.