I think know we can have a theory on why Revenant Chica is wearing a gas mask. So, the theory is that originally the area of Subject 5 Chica (that`s how Chica is called on night 5), was original for Revenant Chica, and employees tested if animatronics can recieve any oxygen, or if animatronics can die from high toxicity meters, tests were deemed unsucessful and Revenant Chica killed one of the employees and took employees gas mask, and Revenant Chica was placed into an interview room, and then she was locked in some random chamber. And Subject 5 Chica was a recreation, because they wanted to try out again, this time it was proved sucessful. The evidence can be delievered just by the existence, of a gas mask on Revenant Chica, plus on night 6, they go away once they see the doll, almost like it just supposed to show us that both Chicas are related, and because Revenant Chica comes VERY CLOSE like she did it before, giving off that proffesional vibe, while Subject 5 Chica, she doesn`t even come close, because she is nervous of doing it for the first time. BUT HEY, THAT`S JUST A THEORY. A FANGAME THEORY, AND goodbye Matpat.