Any who doesn’t have regularly lock their phone obviously does not have kids.
Wrong. Got two. But it is locked now, though not because of them. It was unlocked last year. They are in early elementary school. I’m not sure what problem they would cause with my phone. They have their own electronic devices to keep them happy.
Never have.
I have an Android phone. It displays my carrier on the lock screen, as well as text that has my name plus “if found, please call/text” and my spouse’s number.
You can also pull down the shade on the lock screen, which also displays the carrier.
I don’t think my carrier has my phone’s serial number, but if taken to their store I’m sure they could track me down from the sim card, which is linked to me.
Back in post #14, I linked an article that says Apple won’t/can’t help.
Found an iPhone in an alley about ten years ago. After I charged it, I managed to contact the owner and let him know I found it, and was willing to work out how to get it to him (he wasn’t local). He had already replaced the phone and said I could have it, just as long as I erased the content. No problemo.
OTOH, a friend of mine lost an iPad mini on vacation in Britain. A week after he got back home in California, he gets an email from a guy in Albania asking for the password. My friend would have been more than happy, if the Albanian paid him for the cost of the iPad. Alternatively, he’d be happy to ship the iPad home if paid for that. No agreement was made.
About a month later, my friend gets another email, this time from a Bulgarian student at the University of Palermo in Italy… She apparently bought the iPad from the Albanian, then discovered it was stolen! Well, second verse, same as the first.
If I found any sort of iGadget, I would be hopeful that some poor soul had decided to free themselves from their Apple addiction. 
… and then I would regretfully follow the advice in this thread about trying to return it to the owner. Being unfamiliar with the multitude of arcane symbols that appear on the screens of iGadgets, I would probably end up consulting with my offspring. This isn’t a matter of me being old, it’s a matter of deliberate ignorance. My mother uses an iPhone and a MacBook, and sometimes asks me to help her with them. I hand the problem off to my kids. 
(There are numerous subjects for which I do not reserve any space in my brain: televised sports, Kardashians, Apple, etc.)
If I found any cell phone in the snow beside the road in the middle of nowhere, I would wonder if it had been flung from a moving vehicle by a kidnapper or carjacker!
My experience with a stolen iPhone. Found it on “find my iPhone” and called the relevant city police department. They would not do anything.
(sitting in entryway of Veterans Hospital, waiting for the shuttle to take me to my car. Man came into the entryway, pretended to faint and fall against my left side. Man sitting next to me on my right took my phone out of my open purse. I dropped my purse when I tried to keep the other man from hitting the floor)
Another, though slightly “sneaky” way to identify the owner is to remove the SIM and see if there is any identifiable data on that, by putting it into a different phone. These days with huge storage on the phone itself, few people put much data on the SIM, but you could be lucky.
In my case I picked up an iPhone in a fairly remote hiking trail, it did have some power. My plan was to take it home and fully charge it, but fortunately the owner’s sister phoned it and I was able to answer; and make a plan to meet the owner nearby after my hike.