Secret Door is a Battle Sticker that is seen in Paper Mario: Sticker Star. These special types of Stickers are only used to open secret passageways in certain levels which usually leads to coins, Hp-Up Hearts, and Thing Stickers. In some levels, there are outlines of a door that the Secret Door sticker can be placed on. Secret Door stickers can only be bought at Outlook Point, though, Mario can get a free one from defeating the Shy Guys that were harassing a Toad in Bowser's Snow Fort. Also note, Mario can use Secret Door stickers to run from any battle without fail.
Secret Door stickers resemble blue doors with a checkered-like pattern on it. They also take up four regular stickers in the Sticker Album.