I have a question. Lets say you find an iPhone prototype in a bar and are 100% sure it belongs to Apple. Can you give it to a third party, bartender etc. without being liable if something happens or should you give it back to Apple directly?
Probably the best bet is Apple directly or the Police.
From a criminal law standpoint, giving it to the manager of this type of bar I'm sure would have been fine, no matter who you thought it belonged to or if you had no idea. Bringing it to the police is fine too, and has the added benefit that if you also do some other things, you could end up with the phone one day if the owner couldn't be found.
I'd be wary about giving it to any other third party, especially if you had no idea who they were. If you wanted to to that, you'd be better off just leaving the phone on the stool--you have no duty to deal with it.
This, I have to tell you, is my opinion of what would be reasonable and just efforts. Although I believe that virtually all lawyers would agree, the law is not any more specific than that. There may be cases--I haven't looked for them---that addressed some broadly similar situation as a matter of law, but since each scenario is unique, I'd be surprised if you'd get any help.
I'm highly confident, though, that if you act like you have a genuine desire to get the phone back to the owner, and if you really aren't interested in benefitting yourself from what you found, you'd never have a problem.