Where is the Riddler in Arkham Asylum?

I took all the trophies and enigmas. After the last enigma I got an incoming transmission (if I understood, it was to tell me where Edward Nigma is located) but I didn't understand perfectly.

On the map there is nothing. So where is the Riddler?

My understanding was that he wasn't actually on the island, and when Batman collects all the riddles he deciphers the location and sends the police there. I recall hearing Edward Nigma being arrested over the radio transmission.

A slightly disappointing conclusion to the long search for riddles, perhaps :)

I believe that the enigma has been freed from Arkham island as his cell is empty, but freshly grafittied . He devised a plan that can be controlled remotely, so he can see what secrets The Batman solves from Gotham if freed from custody.