The Secret Door is a very important sticker in Paper Mario Sticker Star. They can first be seen in 3-6 Outlook Point, and this is the only place you can buy them. There is a Secret Door you can obtain in level 4-2. There is a Secret Door spot in almost every level, usually containing hidden Things or stickers. The location of aevery Secret Door location and what's inside can be found on each level's article here, but a total list can be found below. These stickers can also be used to escape a battle.