Here is CA's statute on it.
CA Codes (civ:2080-2080.10)
Poster, it had to have been seen when it fell out to be picked up that quickly, therefore the "rightful owner", you, could be "readily identified", so any other act but to give it back is a violation of the Civil Code.
This conforms with my state on lost property. If I see person drop a wallet, I KNOW who the rightful owner is, if I keep it, it is theft.
If I find a lost wallet, lost, not mislaid, there is a legal difference, if there is no ID in it, then the law follows another course for me. I can use due diligence to find the person, etc., but am not required to go to great lengths to find the owner. Of course, (emphasis is added) to this post, as the case law will point out.