The mask was worn in "This Is A Dark Ride" by Mona Vanderwaal. This person dressed in the same Phantom of the Opera costume as Caleb in order to trick Hanna on the Halloween Train. Mona had the mask under her bed while she was at Radley Sanitarium; which led viewers to believe that it was her disguise. In the Season 4 Premiere, this is confirmed when Mona says she dressed up like Caleb with the Alison mask underneath.
In "I'm Your Puppet," the mask is on the body of "John Doe" when Emily and Hanna search the morgue. It was most likely put there by Red Coat, as she was seen coming in and leaving the morgue by Aria.
In "A DAngerous GAme," Hanna, Spencer and Mona see glimpses of Alison as Red Coat. Mona later tells The Liars that Red Coat wore the mask when she visited her in Radley and during "That Night." Emily and Aria are both unsure if the girls actually saw Alison, or if it was Red Coat wearing the mask.
In "Turn of the Shoe," "A" is seen wearing the mask when she/he attacks Mona in her car.
In "Cat's Cradle," while Emily and Aria search through some of Alison's belongings, they find a copy of the mask with Hector Lime's brand on the back of it. Hanna, Emily and Aria go to Hector's studio and question him about its origin. Hector tells them that Alison had commissioned him to make the mask and to create copies because she wanted all her friends to look like her. Alison told Hector to break the mold when he finished.
In "Face Time," Aria and Spencer follow Melissa to Hector's shop, and they see Melissa dragging a large sack out of the shop. Spencer decides to follow Melissa and Aria goes into the shop. Aria walks to the back room and finds a whole shelf of Alison masks and molds, and finds Hector. He tries to make her leave but finally gives up, saying that he indeed broke the mold, but kept the castings because Alison’s face was so beautiful.
In "The Mirror Has Three Faces," Hanna goes into Alison's old bedroom, and sees Alison's old jewelry box. In the reflection of the mirror inside the box, Red Coat is seen wearing the mask while staring at Hanna, but quickly leaves before she notices. Jessica DiLaurentis tells Spencer and Hanna that CeCe Drake once checked into Radley dressed as Alison. They realize that the masks weren't for them, Alison probably had them made for CeCe.
In "Now You See Me, Now You Don't," The mask is briefly seen when Aria runs after Red Coat inside the factory. They began to fight and in the process, the mask falls off, revealing CeCe Drake. Theres also a "good" Red Coat, who is Alison.