Tested after someone was able to pick up various locked vehicles.
If you lock a vehicle, then add a PIN on said vehicle, it is still displayed as being locked but other players can pick it up as well as perform other actions. Entering a PIN resets the locked state on the vehicle but doesn't update the displayed state!
Adding a PIN, then locking the vehicle works.

To reproduce in multiplayer:
Place vehicle
Lock it
you can see the displayed icon of the vehicle as being locked, yet another player can pick it up.
Visible actions for the other player are fixing it (if damaged), set PIN, fill up, pick up and horn.