You know how sometimes in the acheivment part of the Xbox 360 it says secret acheivment, well ive discovered one firstly in Fort Frolic when you complete the master peice, wait for Sander Cohen to open that locked place with an ability, the kill him and unlock the music box.

Thats just a hint but to get the acheivment, just take a picture of him. The acheivment is called Irony