All three of these items are totally different from an iPod. A car must be registered to it's owner and if you use the keys to drive off with it, the owner will call the police and if you parked it illegally or pulled over, the car will show up as stolen.
A cell phone would have plenty of contacts you could call to find the owner, plus the owner would probably call his cell company and have the phone service disconnected or call the phone him/herself.
A wallet has an ID in it to easily find the owner, plus credit cards would be cancelled immediately.
With an iPod it would be really, really hard to find the owner if it's not registered and I doubt many people would register it, especially someone who isn't even responsible to hang onto it in the first place. My iPod isn't registered and if I lost it I wouldn't expect to find it. Over the summer I left a pair of sunglasses in a locker at my gym, didn't realize they were gone until the next day, went to try and see if they were still there, nope. Realized it was my fault they were gone, didn't deserve to have them, if I'm gonna leave them lying around like that.