If you have an idea of what area the dog is in. I suggest camping out on the edge of that area in such a manner that the wind carries the owners scent into the area. That way, if the dog is in there within close proximity they will pick up the scent and follow it to the owner.
The longer you are present the more scent you leave. If you have helpers , consider having them wear something of the owners, especially if it is a shy dog.