Yunjin Kim (Sun) says that she watches the show, just like a viewer, looking for hidden clues. A couple of days after the shark attack episode (Ep: 2x2, Adrift) aired, she was on set and some of the cast and crew were talking about the Dharma logo on the shark. Yunjin hadn't known about it because it was something that happened after the script was written. The producers came up with it during the editing stage.
Source: Lost: The Complete Second Season DVD - The Extended Experience bonus features.

Go to: Lost Trivia Fact 3