#1) You will need the Archer Costume for this mask. Go south of the hideout, following the coast, until you happen upon a barrel of arrows. Use these to get to the second island, where you'll find the mask behind the tower.
#2) This mask is found on top of the green building in the southwest corner of the northern section of the map. It's on top of a spire jump on the roof.
#3) There is a large bridge that connects the northern and eastern sections of the map. There are bouncy flagpoles on the east side of this bridge that leads to the top, where you'll find the mask.
#4) There is a small bridge in the north part of the northern section of the map. Underneath the bridge is a spire jump, with the mask floating right above it. The flying carpet that goes around the map goes right underneath this bridge, or you can Paraglide from above to get it.
#5) This mask requires either Salim or the Thief Costume. In the central part of the northern section of the map is a very tall tower with a chain that falls apart as you climb it. Either use Salim's Climbing Cobra technique or the Thief Costume's Slow Time ability to climb all the way to the top, where you'll find the mask.
#6) Go to the palace in the northern section of the map. To the right of the stairs, next to the bouncy purple pillow and partially obscured by some plants is a hole you can crawl through that leads to an area named the Lazy Trunk Spa & Lounge. There's a very large purple pillow to the left of the entrance that has the mask hovering above it.
#7) This mask can be found in the Rug Rats mission. Progress through the mission until you reach a point where you have to sneak by a balcony with a flashlight guard, which also has two other guards that don't matter. Sneak by the one, then at the end, go on the balcony and stealth kill all three. You'll find the mask in one of the corners.
#8) This mask can be found in the Copy Cats mission. After you raise the second set of platforms with Murray, you'll have to go through a ship with Sly. Instead of going straight across, follow the debris until you get to a platform with the mask on it.
#9) This mask can be found in the Heavy Metal Meltdown mission. It is located in the northeast section of the map once you're inside. Climb up like you're planting the bug, but go across the chandeliers and land on top of the doorway. From here, you can climb up a short rope to reach two platforms, the second of which has the mask.
#10) This mask can be found in the All Rolled Up! mission. It is located during the actual fight with Miss Decibel. Continue through the battle until you pass the first set of note rails. After this, use the Thief Costume's sword to lower the platform ahead, then go to the one with all kind of crates and containers. On the east side of this platform is a rope that goes down and the mask on a spire jump.
#11) You need to collect all 12 Treasures in this episode in order to unlock the arcade machine in the hideout. Play through the arcade game until you get to the Panzer tank (pink) section. The mask is located in the third section of the Panzer part, where you begin surrounded by rotating electricity. Immediately to your left is a destroyable wall, with the mask behind it.
