The dog in this situation is unable to return home because it has been displaced from its environment intentionally.
Pets in this situation may have been trapped or caught by a neighbor and taken away, caught as a stray by animal control and transported to the local or not so local animal shelter, caught by a local animal hoarder or collector or even been removed secretly by a family member or friend.
However it happened, the pet was removed intentionally and is unable to return home.