I'll try to clarify the issue a bit.
Spotted is a key word. Partly because players may be ignorant of last location someone logs out from, partly because people can be on the server and no where near a ship in question. Being online while a ship(s) are unprotected does not automatically give them immunity. Also a visibly manned vessel would full under Ships and Vehicles attacking rules.
Recently is a reasonable time where one can consider 'Well, perhaps they had to leave quickly and may be back in a few hours. They either didn't feel the need to lock up, or were rushed.'
Now, the last action you did on board was place that cobblestone, two days before the sinking. You did not access the chests at any time while the ship was in this location.


However, as you mistakenly believed that it was locked, I've clarified the rule and have moved the hulk to beside your last recorded action.
Anything further?