SLY MASK #1
The first mask is behind a tower on a island south of the hideout. Use the Archer costume's rope arrows to get there.
SLY MASK #2
The second mask is on a spire atop the large green building just north of the village. It's the same building where you first tail Ms. Decibel.
SLY MASK #3
This mask is atop the massive bridge arch in the middle of the city. Use the springy flagpoles on its eastern side to ascend to the top.
SLY MASK #4
Mask number four is under a bridge in the northern part of the city. For convenience's sake, the magic carpet that patrols the level passes this spot.
SLY MASK #5
Scale the massive tower in the north-central part of the city - the one with the dissolving chain. Use either Salim's Cobra Climb or the Thief costume's Slow Time ability to climb safely. At the top of the tower, you'll find a mask.
SLY MASK #6
Head to the palace at the north of the map. To the right of the palace bridge is a very hard-to-spot hole Sly can crawl through. This will take you to a secret area, the Lazy Trunk Spa and Lounge. To the left of the entrance is huge bouncy cushion with a mask floating above it.
SLY MASK #7
This is found during the mission "Rug Rats." While playing the mission, you'll spot the mask guarded by some gorillas. You can hop over the fence to grab the mask; this will alert the guards and send you back a checkpoint, but you'll get to keep the mask. Alternatively, you can backtrack a bit after crossing the ledge and take out the guards stealthily.
SLY MASK #8
Proceed through "Copy Cats" until you use Sly to pass through a boat that Murray raises from the water. Ride the debris to the bow of the ship to find a mask.
SLY MASK #9
This one's found during "Heavy Metal Meltdown." When placing the bug in the northeastern part of the map, you can climb to some high ledges to the room's south. A mask hides up here.
SLY MASK #10
This mask is actually found during the Ms. Decibel boss fight in "All Rolled Up!" After passing the first set of note rails, you'll break a chain to lower a platform and reach an area with several boxes and poison canisters. On the east side of this platform is a rope. Descend it to spot a mask out in the void you can spire jump to.
SLY MASK #11
Once you've recovered every treasure in the level, the arcade machine in the hideout will be repaired. Boot it up to play System Crack Extreme. Play until the panzer section, and make your way to the third zone. In this zone, there is a breakable wall on the upper left. Bust through to find the level's final mask.
Note: For more detail on collecting the treasures required of this mask, see our Sly Cooper: Thieves in Time treasure locations guide.