Dogs can get lost for all sorts of reasons; even if the most obedient pup can wander off.
The first thing you should do is check for any form of identification - check the dog’s collar or tag to see if there are any of the owners contact details on there. Many people use social media and word-of-mouth to help locate the owner - chances are, they’re trying to find you as much as you’re trying to find them.
If you’re not having any luck, then legislation in most states requires all strays to go to the pound or council for processing. This is so they can be checked for microchipping and so on.
In addition to this, you can always use the PetRescue Rescue Directory to find a rescue group in your area - they may be able to help you out, and advise you on what to do next.