Unless the iPad that you have found displays a message that includes contact details of the owner, you will not be able to trace the owner. Apple will not be able assist with tracing or reuniting the device with its owner.
You would be best advised to hand the device to your local authorities - as you appear to have done - along with an explanation of where and when the item was found. Alternatively, if found at a transport hub, hand the device to the lost and found department.
If the iPad is locked with an unknown Passcode or AppleID - or is disabled - the device is and will remain useless to anyone other than the bona-fide owner.