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. South of the hideout, on top of a nest. You can access this by heading to the hideout's rooftop. There's also a bottle at the roof so you'll spot this while collecting the bottles.
2. Under the wooden bridge to the west. You have to ride a glacier to reach it.
3. Job: Ice Ice Bentley
After stealing 3 gems to open the gallery door from the mammoth guards, you'll have to navigate again to a tunnel with geysers and rolling eggs. Once you've reached the end of the tunnel, you'll find two breakable walls. The one to the left leads to your objective, the other one contains a mask.
4. Job: Jurassic Thievery
After opening the second valve with Bob and before heading to the right, continue further down to find a platform containing a mask.
5: Job: Jurassic Thievery
As Sly and after reaching the part where you need to paraglide across the hot air vents with fiery, pendulum traps, you can find this mask on a separate platform to the left. Refer to the video below for more details.
6: South of the map, on the nose of a large penguin statue head. You have to go around the slanting rock and climb the branch there. Para-glide to reach the mask.
7: (Bob required) This mask is found on the ice wall wall to the west, near the lower wooden bridge.
8: (Bob required) Your landmark is the Grizz's statue to the west. To the south, you'll find a climbable ice wall. You'll find the mask on top of it.
9: Job: Unexpected Ties
After getting the Sabertooth Costume, check the back of the bed to find the mask there.
10: Job: Going Up
Once you've reached the part where you need to climb an ice wall while avoiding the dropped eggs, continue to the top of the first set then head to the right to find this mask.
11: Arcade Mini-game: Spark Runner
After collecting all the treasures in the area, the arcade inside the hideout will be fixed. You have to make the mask appear in the first area. You have to collect all the purple time-ups in the first area then keep bouncing on the electrical bumpers until you hear Bentley say something like “It's like a pinball machine in here!”. The mask should appear in the lower left corner of the starting point.