I've returned wallets and purses I've found on several occasions. The only one I regretted was to a woman whose phone was continually busy. When I finally got through to her she said that she'd been on the phone cancelling all her credit cards ... and then she abused me for not ringing sooner. Truly! That's the kind of gratitude that really warms the soul.
But the classic answer to your question comes from Yogi Berra who was once asked what he would do if he found $1 million in the street. He said, "I'd find out who lost it and if it was a poor person, I'd give it back."