Return the phone.
You have a choice - you can tell everyone you bought it, and lie to them, or when people ask where you got it, you can say "I found it on the bus and didn't return it." Sure, you can say "I found it on the bus and nobody claimed it and blah blah blah" but you're lying and that's a lie, too. And now there's this divide between who you are and who you say you are and even if the people around you can't put their finger on it, they'll intuit the discrepancy.
Or you can just straight up be that guy that takes people's shit. That's fine, too. That means that people will default to not trusting your ass. If you are unaware as to the fundamental crimp that will put in your relationships and the way you conduct your life, have no fear because you're about to find out. Pay really close attention, though. Because the opportunities you'll miss won't be visible to you unless you really think about it. You might not wonder why you aren't watching your buddy's house while he's away unless you think "maybe it's 'cuz of that phone I stole." You might not think about why you weren't part of the pool that bought Phil a new pipe when his old one vanished unless you consider "maybe it's 'cuz my friends know I steal shit and they think I stole the pipe."
i don't care what fucking phone it is. Take the price and knock 50% off it 'cuz that's what it's worth used. Now take that number and knock it by half again because there you are, "half-excitement and half-guilt." Maybe it's a fully loaded 6S plus. That's what? $900 unlocked? Congrats. You just invited this bad karma into your life for $225.
I couldn't tell you how many phones I've returned. More than a dozen. The hole you put in someone's life when their phone disappears is formidable. Everything you have is suddenly unsafe. All your data is out there. My wife had her phone lifted off the sidewalk within 30 seconds and she had the data for 3 A-list celebrities in there. So she had to call publicists and say "guess what, your data might be out there" even though she had to scramble and wipe it remotely. Fortunately I kept the guy busy by offering him $200 cash no questions asked if he just met me at the airport, where he claimed he was going anyway. $200 for a 3-year-old base-model iPhone, mind you.
But the kind of people who steal iPhones? They're too busy dealing with the guilt of "oh shit maybe this time they'll bust me" than think about the fact that they just made an easy $200 by eventually doing the right fucking thing.
Right now, you're on the path to being one of those bottom-feeders that steals phones. You're well on your way to being the guy who thinks he's going to jail for pocketing an iPhone. That's because the people who steal phones? Fuckin' A - they're out of friends.
Because friendship is trust, and from this point forth, you're going to have to square what you did with who you know and every bit of sanding and buffing and polishing is going to make you smaller.
Return the phone.
BTW - know what I got in my kitchen? Powered down and unused? A Nexus 6 Plus. Know why? 'cuz a friend gave me his spare when I bricked my OnePlus. I used it for a while, then went back to the OPO, 'cuz the Nexus is too big. But it's still there. See, when you're the kind of guy who returns phones, the universe sort of arranges for you to have phones. I'd send it to you, but...
No, I wouldn't. You're the kind of guy who steals phones.
Return the phone.