Maybe not in common law, but California State Law is pretty clear on the matter. If you find lost property, which the iPhone prototype was, you are duty bound to turn it in so it can be reunited with it's owner.
You're misinterpreting this a bit. You don't have an obligation to return an item that you find; you can always leave it where you found it. You are obligated to return the lost item only if you take it.
Informally put, the law codifies the idea that returning lost property to its owner is a favor that you can do them. But as such, the only things you can do with lost property are those that you can justify as helping the owner reclaim their property. So, for example, if you take a guy's lost phone to return it to them, you can inspect and operate the phone to identify the owner—and even call the owner using their own phone to tell them that you found it—but you can't use it to go on a 900 number phone sex orgy at the owner's expense.
Another example: you're generally forbidden to sell lost property, but there's an exception that if the property is perishable, you may sell it and later give the proceeds to the owner. Again, the reasoning is that selling the items before they spoil is in the interest of the owner, so you're doing them a favor by selling it on their behalf.