The dog owner should take a piece of clothing that they've worn all day, so the dog can pick up their scent on it. Place it at the location where the dog was last seen. If the dog has a crate or favorite toy, you can leave that too. Leave a bowl of water (but not food). Leave a note asking people not to move these items.
Check the site regularly. Hopefully the dog will be waiting there. Good luck. (Original photo by @TheWorldImages)