strange this piece kept buoyant crossing the entire indian ocean !
Not strange at all.
The CofG of this flapperon is hardly likely to be symmetric, and so it would not float in a flat position. It would float vertically, with a small pocket of air trapped at one end. And in that position, it would be quite stable and quite invisible from the air.
And if someone had pulled off a successful ditching, it is likely that the only debris would be the flaps. And that is what has been found.