I foster abandoned or strays until we can find either the parents or get them adopted. Tags and chips are a must! It's amazing how many pet owners don't chip or tag their pets.
The head of our foster group just brings over her scanner whenever I get a new foster dog in. In all instances when the dog was chipped or had tags we were able to get them back to their owners.
The couple of abandoned ones I've fostered had no chips or tags and the owners were never found. This includes the two puppies that were thrown from a moving vehicle.
If you are having someone watch your pets when you are out of town have a new tag made that has their phone number on it in case they get loose.