The person returning a wallet has protected several inconveniences they will never have to hear about.

A returned wallet prevents a chain of trouble that nobody ever describes. The cancelled cards, the locksmith, the day off work, the photograph that could not be replaced simply fail to happen, and whoever prevented all that never sees the version they cancelled. Most decency works this way, with results shaped like absent events.

Do the small returning thing even when it is inconvenient and goes unthanked. And when something is handed back to you, say the whole of what it saved rather than offering a quick thanks. People rarely learn the size of the good they did.