Apple Signs are the game-wide collection players are tasked with completing in The Thief and the Tinderbox and its accompanying bonus game, Journey of Atonement. There are 30 symbols hidden throughout the main game, with one found in each and every location of the game. The remaining 9 are found in the bonus game. Finding all 39 symbols will earn the player an Achievement entitled "Sign Collector".
Underneath the Map Button in the game's interface, there is a small apple button. Hovering over this button will tell you how many of the symbols you have found thus far. This button is purple whenever you're in a location where an apple can still be found. Once the symbol has been located, the button will turn gold. This mechanism can be used to help the player find any apple symbols they may have missed before completing the game.
If the game and bonus game are completed without the player having found all of the apple symbols, the "Collectibles" option on the Bonus Content menu allows the player to return to any locations from the game in order to pick up the symbol the player missed.