The Roxy Mask is one of the collectibles you can find while traversing the pizzaplex. This item is hidden amongst others when you reach Roxy Raceway. To get there, you will need to avoid getting caught. Here is where you can find the Roxy Mask in Five Nights at Freddy’s: Security Breach.
After you have progressed a good amount in the story, you will get locked in the lost and found. After you escape, you will need to make it to the elevator while trying to avoid getting caught by Vanny. Once you are on the elevator, Freddy will tell you that you must climb the boxes leading to Roxy’s Raceway and he will meet up with you.
When you exit the elevator, head to the left and go through the construction area leading to Roxy’s Raceway. After you climb the boxes, you will see that Freddy is in danger. Instead of running down to him, stick to the right side of the area. You will find some scaffolding. Run up to the top of the scaffolding to find the box that contains the Roxy Mask. You can also obtain this item after talking to Freddy.