There is a tall nest immediately behind the hideout. The first mask is atop it.
Head to the south end of the map and get atop the large teetering rock. Look further south to spot this mask out on the nose of a large totem head. You'll have to take a leap of faith and paraglide out to it.
Now head to the icy river where the ice chunks flow. Ride one beneath the southernmost bridge to find a mask underneath.
Immediately on the western side of the same bridge is an ice wall Bob can climb. Shimmy across to find another mask.
Head to the ice sculpture of The Grizz on the western portion of the map. Just south of it is another ice wall Bob can climb. At the top is another mask.
In the mission "Unexpected Ties," proceed until you reach the Sabertooth costume draped over The Grizz's bed. Behind the bed is another mask.
This is found during the mission "Going Up." Proceed until you climb past the first pair of falling eggs. Rather than going left, go right to find a mask along the ice wall.
During the mission "Ice Ice Bentley," you'll move through some ice tunnels where eggs roll. In the second tunnel, toss a bomb at the cracked wall across from the tunnel exit. Blow the wall to reveal a mask in the alcove beyond.
You'll find this mask during "Operation: Jurassic Thievery." When taking out the pressure valves as Bob, proceed until you release the second. Drop straight down past all the flames to find a lower wall to climb. Head right, hop the spike pad, and drop to the ledge to find another mask behind some crates.
In the same mission, play until you reach the part where Sly must paraglide across the air jets and take out the switch guard. On the way, look for this ledge. There's a mask on it.
Once you've recovered every treasure in the level, the arcade machine in the hideout will be repaired. Boot it up to play Spark Runner. The tactic for getting this mask is strange, but here it is: gather all the purple time boosters in the very starting area. Next, bounce around on the bumpers until Bentley says, "Geez, it's like a pinball machine in here!" Once he says that, a mask will appear next to the silver lining to the left of the starting area (immediately above the red hole in the floor).
Note: For more detail on collecting the treasures required of this mask, see our Sly Cooper: Thieves in Time treasure locations guide.