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. Inside the Imperial Prison, during the job “Breakout”
After speaking with Rioichi Cooper, follow the path until you find two guards. Go where the tortured monkey is and you should see a hole with blue sparkle. Unequip the samurai armor and crawl into the hole to find a mask.
#2. Inside the Imperial Prison, during the job “Breakout”
On your way to Rioichi cage and after ziplining down to the second prison area, make your way up and head to the right so the camera angle changes a bit and allows you to find another hole with blue sparkle. Crawl inside to get the mask you saw earlier.
#3: Job: Sushi House Startup
As soon as the mission starts, look to the corner to the right to find this mask. You can reach it easily by using Rioichi's Dragon Leap.
#4. Job: Sushi House Startup
After collecting the third knife and opening the door, don't interact with the marked lever first. Climb the pole to the left side and move your camera around to find a mask in the corner.
#5. Job: Pretty in Pinker
After crossing the pond by performing timed dragon leaps, go to the upper level and make it across the room using the ceiling fans. Destroy the box in the corner platform to reveal the mask.
#6: Job: Something Fishy
After the hacking mini-game, turn around to find this mask by the door.
#7: Job: Altitude Sickness
After seeing the airship fly, turn around to find another mask.
#8: Job: Altitude Sickness
Cross the flaming bridge by jumping from one point to another. Continue to the large gate but before opening it, turn to the left to find another mask.
#9: Northernmost part of the map, on a small cliff across the chasm.
#10: Westernmost side of the map, behind the golden rhino statue
#11: Arcade Mini-game: Alter-Ego
After collecting the treasures, the arcade machine will be fixed. Play the mini-game Alter Ego. In the first stage, keep fighting your way through until you spot a portal in the middle of the stage. You must enter the portal before the screen starts to scroll down or you'll get locked from it. You'll have to restart the checkpoint to access it again.