One person returning the lost phone makes the next person a little more likely to do it too.

What people do is shaped by what they believe other people do. Hearing about one person who handed something back adjusts that picture slightly for everybody who hears it, and the picture is what most of us check before deciding. Honest acts are quiet, so they rarely get counted, and they still move the baseline.

When you do the decent thing, you do not have to announce it, and you do not have to hide it either. If somebody asks how the wallet got back, say what happened plainly. You are part of what the people around you assume is normal.