The key on Serrick's island is the joker one (not opening up an additional lvl) - and as long as the wine chest Desdemona mentioned will not open with the key in the torch - no one will ever know if key 5 is in there or if there was once a key on Serrick's Island opening lvl 5 and taken out of the game.

In any case => I consider it strange that Serrick has no proper BF key, as all the other island parts of Act 3 had one.
Kiya