Secret Safe

The Secret Safe is the last ship part that can be retrieved in Pikmin. However, it is not necessarily a part, because it has no purpose in making the Dolphin function. It apparently is a large safe, obviously shaped like a pig, better known as a piggy bank. This is where Captain Olimar keeps all of his money, or Pokos. When he found it, none of his Pokos were lost.

Pikmin: Connection of the Worlds
This plays a very minor role in Pikmin: Connection of the Worlds, and is actually never seen in-game. However, at the start of the game, Sagittarius mentioned that the Secret Safe helped upgrade the Dolphin.