The last thing you should ever do is give it to the office, they'd take it (though I know it's either you keep it or you give it to the owner now). Years ago in middle school, my friend and I had GBAs out at lunch and they took it from us and said they'd give it back. Next day, we ask and the same person who stole it said they had no idea where it went. We didn't do anything for a while, and later searched the entire office and found them on top of some drawer, luckily..
And one of my friends also lost his ipod in highschool, and the school found it, and he knew it was his but his name wasn't in any part of it, despite the fact that he had the exact combination of music from it. He never got it back >_>.
Yeah anyway keep it and if that person doesn't say anything, then they probably don't care and have got another, and would also probably not appreciate it if you gave it back..and it's their fault they lost it, but most of the time, I would give something back.