Overview of collectible masks for Arthur (20 Masks) - Sally (12 Masks) - Olli (12 Masks). The mask names are translated from the German version of the game.
Arthur - 1: Promises
First safe-house (hatch) in the game - Barrow Holm.
Arthur - 2: Lies
Church in the small town Borrow Holm after fleeing from the madman.
Arthur - 3: Uncle Henry
In the crashed plane at the safe-house (hatch) Eel Pie Holm - at the garden.
Arthur - 4: Battleship
On the second watchtower, on the Inkerman Bridge on the way to the militarybase - Eel Pie Holm.
Arthur - 5: Up A Tree
Access to the military base via side entrance, just before the bunker.
Arthur - 6: Scouts
On the military base in the building with the inscription canteen.
Arthur - 7: Rain
In the Dogberry Park on Maidenholm - over a small pond (park with rotating heart - side mission "Love Birds").
Arthur - 8: Children
Police Station Maidenholm - second office upstairs (next to archive department).
Arthur - 9: Caning
In the reform club, in the room at the "receptionist".
Arthur - 10: Underground
In front of the entrance to the Safe-House (hatch) Luds Holm - (former subway entrance).
Arthur - 11: Latin
Dr. Faradays House, before going up the stairs - Luds Holm.
Arthur - 12: Germany
Newspaper office - second floor, at the end of the desks - St. Georgs Holm.
This is a bug - I can collect it, but it will not show up in the overview.
Arthur - 13: Yellow
Thomasins House - behind the curtain in the kitchen - St. Georgs Holm.
Arthur - 14: Clothes
St. Genesius Church - side mission "The Church of Simon Says" - between the bookshelves.
Arthur - 15: Swimming
In the sewer, after driving with the jubler. Where you have to activate three power switches to escape (in the room with the first power switch) - St. George's Holm.
Arthur - 16: Sally
In Dr. Veluce's Office - Haworth Labs.
Arthur - 17: Trains
Mount Badon Bridge to Parade District behind the front door (Quest "Letter of Transit).
Arthur - 18: Chess
Director Ringwells Office at Wellington Health Institute, on the right side in the office.
Arthur - 19: Eureka
On the mien, head office where you activate the crane control.
Arthur - 20: Home Again
London Bridge, after escaping from the mien, between the trains.
Sally - 1: Brave
Magiddo bridge to Luds Holm - right in the corner after the exit.
Sally - 2: Gingham
At the playground in Luds Holm - where Arthur and Sally meet.
Sally - 3: Plan
Safe-House (hatch) Rattenholm - if you open the locked door on the right side on a few boxes.
Sally - 4: Beastly
Dogberry Park - at the fountain with rotating heart.
Sally - 5: A Girl Like You
Plassey Bridge towards Maidenholm - behind the chopped door on the right.
Sally - 6: Rue
Directly at the mission of the witch sisters (wedding stones).
Sally - 7: Delicate Flowers
At the train station Luds Holm - at the mission where you should catch the 10 butterflies.
Sally - 8: The Best And The Brightest
Right in front of the Joy Factory - Mission go to Dr. Verloc to get the cod liver oil.
Sally - 9: Posh
In the boathouse on the military base - left side of the boat.
Sally - 10: Preggers
In the safe-house (hatch) Luds Holm - where the infected ones made themselves comfortable.
Sally - 11: What Boys Want
Julia Chaney's house - in the bathroom behind the shower curtain - Sidemission where the 3 proofs must disappear.
Sally - 12: Different
Safe-House (hatch) St. Georgs Holm - directly above the chemistry table.
Olli - 1: Dancing
After Olli has fled the train station roof - right next to the broken entrance to the train station on the small wooden walkway.
Olli - 2: Hildegarde
Under Johnny Boltons Shop in Luds Holm.
Olli - 3: Question
On the Inkerman bridge towards the military base, in a small dungeon on the left.
Olli - 4: German
On the Wednesday hill in Luds Holm, about the side mission "ghosts".
Olli - 5: End
In the Safe-House (hutch) on the military (Ravensholm), in the toilet area.
Olli - 6: Playhouse
In the fashion house (Clyton Center of Art and Design) in the Parade District, in the Dressing Room (after going up the hatch).
Olli - 7: What Jack Did
In the Jack o Bean Club on the 1st floor in the bathroom.
Olli - 8: A Simple Plan
Parade Destrict - access from the lower area to the upper area - towards the Department of Economics and Research - directly at the metal gate.
Olli - 9: Parade
In the Safe-House (hatch) Luds Holm, right next to the chemistry table in the small passage.
Olli - 10: Sally Boyle
In the Broadcast Tower on the second level (at the left waterfall) - where you turn off the control for the "secretary".
Olli - 11: The Martyrs
On the Megiddo / Salamanca Bridge from Maidholm direction Luds Holm on the left side just before you enter the tower.
Olli - 12: Mama
The first main mission with Olli - the mask is in the building where you have to free the prisoner, on the second floor a little hidden in the corner.
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Source: https://gameplay.tips/guides/2823-we-happy-few.html