If the local police chief has an unidentified vessel on his patch, I'd argue that he has every right to go and take a look at it - he could do worse than transmit his intentions on guard since presumably the ship will be monitoring that, although that itself may be problematic since (a) he may not have a formal police callsign, and (b) even if he does the ship may not recognise it.
I think the police chief could carry a loudhailer and shout a warning such as "Police - Go away or I shoot!"