As Daube said, it's in the "Tokyo Executive Transport" mission. I think it'd be more fun to fly the mission and see what's there, and take notes if I wanted to find the island again.
"I reject your reality, and substitute my own" Adam Savage, Mythbusters
Congratulations for your semi-sucessfull attempt, Boikat Unfortunately, as far as I remember, the secret Island is only visible in the mission. If you try to get there on freeflight, you won't see a thing... :/