The Seychelles are a group of islands in the Indian ocean.
That depends on the location of the island itself.
The exact location is Ontario .
San Nicolas Island
somewhere
Crete Island ..
Vancouver's relative location
The main reason is probably because it doesn't matter what the island was called. If you knew precisely which island it was you would also know its location and this would distract you from the essential story. The location and name of the island are irrelevant, what matters is what takes place on the island, and this could happen on any island or any other remote location.
Cape Hatteras Lighthouse Location: Hatteras Island -->credit: thank the azn.
There isn't a location called Mystery Island.
It is.
Peru
If you are referring to a location, either form can be used, as long as you know what island you are talking about.