Right now there is only a few ways for a ship to be "stolen"
1) They are assigned pilot/commander and the person docks it, making it inaccessable to you and can transport it to other locations where you will be unaware, unless you know where there ship is, or if they finally chose to undock it
2) You make over the ship to the said person
-If they are assigned pilot/commander, they could pilot the ship, but you could still unassign them and they won't be able to do a thing and will be stuck. However they could "sun" you ship, in most cases, it will result in complete destruction of your ship.
3) Lockbreakers don't work in the way your thinking about it quite yet. The only way for one to board a ship is if they are assigned the ship "Crewlist, Commander/Pilot", within the ship itself there can be doors where the locks are "existent". There isn't a "door" to enter a ship quite yet, though it could be implemented in the future.
As Zero said, someone coming up to your ship and hopping and and stealing it doesn't work. Until we have combat where you can react and get back at them, stealing like that won't work (at least with ships etc, items and weapons you can pick up and use without having ownership).