Danielle scientific exploration boat crashed on the island. She hears the others because she hears the whispers.
I know this is the rational explanation but, this is more fun and a better story.
Danielle's ambiguous military/scientific background and training have never really been discussed. Partially because she's a total loose canon.
We don't know who Rousseau was doing research for... French navy, i doubt it. Independent or government sponsored scientific research company? Dharma, Widmore, Hanso, WHO? We've never found out exactly who sent her there. Sayid noticed her jacket looked more like a soldier's than a scientist's. What was her crew doing in the south Pacific?
She says her crew heard the numbers and crashed on the Island. We've never seen a ship. She says she killed her whole crew. Where did she bury them? Can she draw a map for Sayid? She said that they were all infected. We've never seen a disease.
She may have crashed there but was it accidental?
ROUSSEAU KICKS A$$! Can't wait for a flashback!