They are almost impossible to unlock if it’s not your own. The FBI just had this problem not to long ago here trying to gain access to a suspects phone. Apple would not help them break into it either. You best bet is to see if someone reports it lost. (they may offer a reward) or tun into the local authorities. IF it was found outside a business, turn it in there. Someone may already be looking for it or it may belong to an employee that works there.