Masks are scattered in various missions and episodes. Collecting them will allow you to enable extras (accessible from the start menu). If you missed a mask, you can always go back and replay the job/ episode to get them.
#1. On the roof of the tavern, northwest of the map.
#2. On the lower “branch” of the tower northwest of the hideout.
#3. Job: Juggling Act
After shooting the moving target, you should be able to spot this mask from afar. Just shoot the target beside the mask to reach it.
#4. Job: Juggling Act
After jumping across the second set of frog trumpets, you'll have to grab hold of a dangling rope. Before jumping to the next platform, continue climbing up instead to find this mask.
#5. Job: Eye in the Sky, Southeastern tower
Required: Sir Galleth
You can get this mask the first time around during the job “Eye in the Sky”. After breaking the first vertical barrier, make your way to the third ring and peek to the ledge farther to the right to find this mask.
#6. Job: Eye in the Sky, Middle, south tower
Required: Sir Galleth
As part of the job “Eye in the Sky”, this mask is located in the topmost ledge between some crates. It is hard to miss.
#7: Job: Shell Shocked Heart
Once inside the blacksmith shop and after deactivating the first set of lasers, continue following the path until you find a large gear. There's a mask behind the wooden board so blow it open using your bomb to get the mask.
#8: Job: Shell Shocked Heart
After successfully hacking the second terminal, continue forth. Before turning to the right, jump on the rotating gears to the left and destroy the box in the corner to reveal another mask.
#9: (Sir Galleth required) This is located on top of the building northeast of the clock tower. Make your way up to the iron bars in the upper part of the building and move around to find a ring where you can use sir Galleth's catapult crash.
#10: Job: Mechanical Menace
Once you've reached the large machine at the end of the tunnel, peek over the edge to the right to find the mask there.
#11: Arcade Mini-game: Super Alter Ego
After collecting all the treasures in this episode, play this mini-game in the arcade. You have to enter the optional area through a portal that's found in the lower right portion of the second stage.