A stranger found your wallet and mailed it back with every card inside. Pass that energy forward at least once.

An anonymous kindness breaks the rule you were quietly operating under. Somebody with no stake in you, no chance of thanks and no way of being found went to real trouble, and the effect is larger than the wallet, because it revises your estimate of how people generally behave. That revision is the thing that actually got returned.

You cannot pay it back to them, so the only move available is passing it on. Hand in what you found, give the directions, take the extra minute. Do it once without telling anyone and you will understand what they got out of it.