My friend gave me a iPhone 5s without a sim card. She found it and whoever it was they didn't report it stolen. Can I just go to Metro PCs and connect it there? I restarted it and it said sim card required but no-one has reported that it's stolen and I already have a line with Metro PCs. She said I could switch it and get a sim card it would cost 30$. It didn't have a password or anything. She was using it just fine but then her mom got mad at her for having it. She tried locating who it belongs to but no-one came up.
Since your friend used it without problems, there would have been a lot of information on the phone for who is the owner. In palaces like Contacts, Messenger, Calendar, Email and so on, you name it.
Call any of the Contacts listed and ask who's phone this is you are calling from.
Follow her moms advice be a good person and return it (just imagine you lost the phone).