Board Thread:Sly Cooper: Thieves in Time discussion/@comment-50.27.22.78-20130210012451/@comment-5876466-20130211040420

It's in the boss fight with Ms. Decibel. After confronting her the first time, running along the pink music ribbons, and then confronting her again, you will need to use the Thief Costume's sword to cut a chain that will let you continue on. When you come to the second chain, there should be a rope hanging over the edge near it. Climb down there and the mask should be there on a spire (you might need to turn the camera a little to see it). Enjoy your platinum. :-P