Your legal obligation is to call it to his attention, not to take it and put it in your pocket.
When this neighborhood was new we had several families from Latin America move in. They came from one of those countries where anything you don't have locked inside your walled hacienda is fair for the taking by any and all passersby.
That's their law. It took some work but I got them turned around before someone shot them. Most folks here are armed to the teeth so there was a real risk.
You sound like you are well-grounded in the doctrines of an alien culture that's not compatible with our Common Law or our statutes and traditions.
You don't see some guy drop a note.
You are walking through Time Square. Huge crowds. You see a $50 note on the street. You pick it up You haven't stolen anything.
Now assuming that after repeaed attempts to return said money to the allged owner, the alleged owner repeatedly refuses tio accept the money, and wouldn't even acknowledge that they lost any money at all, the money reverts to you the finder, after a specified time period.
When the finder of this Apple smartphone repeatedly tried to retrun it to Apple, and Apple repeatdly refused to accept it, and wouldn't even acknowledge that they lost any phone at all, that's it.
No theft was commited anywhere.