I remember a time when I used to work in a phone store that repairs off-warranty phones 2 years ago.
There was an iPhone 4 that was fixed but never claimed since January 2011 and calling the owners lead to someone else. Apparently the original owner changed his/her number. Since the iPhone was wiped and it was just sitting there. I took it.
The store has a policy of "If you don't claim your phone in 6 months it becomes store property".
So basically, it all comes down to your own morals. 2 years ago I didn't have much. Not even enough to even ask my boss about it since it became store property and I "technically" stole from the store. (Thing was in the fixed phones box which is opened to remind and call customers about their fixed phones nearly all the time).
I'm actually surprised that my boss never asked about it but then again, I'm pretty sure she'd suspect another worker there instead of me.
In your case, if your boss isn't going to do anything about it, think about it this way, if you hand it over to the police, what could they do better than you? assuming they can/will do anything at all.
It's a misplaced/stolen ipad. EASILY a cold case.
If you don't mind troubling yourself, sneak it out and try contacting/finding the owners yourself. if your boss asks you about it, just say you tried to contact the owners yourself. If not, do what you wanna do.