Found 5s iphone, how to find owner if it was restored?

So apparently my oma found an 5s iphone in her lawn a few weeks ago and brought it to my household. My sister tried to find out who's it is but couldnt because it had a pass-code. They tried to call the police to see if they can find the owner but they said they would be unable to help, so my sister thought if she plugged it into her computer she could access stuff that way- that didn't work so she restored it in hopes they could somehow reactivate it and get in w/o a pass-code and see if she can find the owner that way (she's obviously not very bright when it comes to electronics.) Now fast forward to today and she hands me this phone telling me to try to find its owner (the last thing we want is for someone to think we stole it!) is there anyway to find the owner? If not, is there a way to just make it so we can use it? Finders keepers, losers weepers- the last thing we want is some business taking the phone and making (another) profit off of it when id much rather have the owner get his/her phone back! (but if not we want to be able to keep it) Keeping it is the last thing i want to discuss though, please tell me there's some way to get this to its owner?

Easy, take the phone to the cell phone carrier and they can look at the IMEI or ESN and tell which carrier it was bought through. If the phone has a SIM card, you can pop it out and the carrier will be printed in it. Once you get it to the proper carrier, they can use the IMEI number to identify the original owner, they can give said owner YOUR contact info, and they will ask them to get in touch with you if they wish to.

Since most people don't just randomly lose phones in a random yard (unless drunk out of their minds), I'd assume that the phone was in deed stolen, reported so, and is not locked so that it can't be activated on a carrier or iTunes/iCloud, making it a paperweight that looks like an iPhone.

And most folks would think that the last person with the phone, was indeed the person the "stole" it.

Go to the carrier of the phone and they can probably unlock it or use the serial number to locate the person. If you keep it and the person is still looking for it by turning on "find my iPhone" they can press charges against you. (Happened to my friend)

Call the number.