How do you find a missing dog?

A friend of mine knows that I have joined Fluther. And today, she had requested me to ask you guys for your valued opinions. Help me help a friend? =]

5 of my friend’s dogs got out of the house. 4 were recovered. 1 has been missing for a week now. My friend and her family have been searching day and night for him. They had already reported the incident to the local police and had asked around – including the pound, Animal Society and vets. She has given out flyers and had asked children, who frequently play outside, to be on the lookout.

Do you have any more tips on how they could find the dog? What are the chances of him eventually coming back home on his own? Considering that isn’t wearing a dog collar, he’s always inside the house/behind the gates and the only times he was out was when he tried escaping (twice). When do you start considering that you might actually not find him anymore? (My friend dreads the answers to this last question though…)