I'm not sure you can get into all contacts without getting through the lock screen, even through siri. You could speak to siri and say contact and then say a random first name , that should bring up a list with all contacts with that name and you can call/message them. Siri wont be able to say who owns the phone though.
However, messages, emails, and notifications should still appear, if you swipe upwards there might be some still there. Could give you some way of identifying the owner or contacts that you could contact?
Otherwise just hand it in.