Generally I'm talking about ships that are in use and landed on a planet. You can walk onto the same square as them (if you get the lockpick roll), but you can't go inside them and steal them blind. :P
I think you should be able to go inside them, although that would generally count as piracy, and have the usual options when inside someone else's ship.
![smile :)](./images/smilies/icon_e_smile.gif)
Larger ships would generally have some security force and / or traps left behind for 'visitors'.