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. After reaching the museum, head to the roof. Don't go to the upper roof for now. There's a mask nearby that you can grab. Climb on the crate to the left to reach the roof and jump to the other side, behind the fence.
#2. While controlling Bentley and after destroying the large gear along the way, jump to the other side. Before moving further, check the tunnel to your immediate left to find the mission's second mask.