I found $400 in cash on the sidewalk. No one was anywhere nearby who might have dropped it. About a week later, after I’d spent at least half of the money, I told a friend about finding it. She said she’d seen signs posted by the person who’d dropped it on the last day of a visit to San Francisco, where I live. That person has since returned to New York. I soon enough located one of the signs. Am I now obliged to contact the loser and give it back?