Clue bottle

Clue bottles (or clue in a bottle/clues in bottles) are collectible items found in Sly Cooper and the Thievius Raccoonus, Sly 2: Band of Thieves and Sly Cooper: Thieves in Time. If all of the clues are found in a level, then Sly Cooper (or occasionally another playable character) will gain access to the vault (or safe). The number of clues varies from level to level in Sly Cooper and the Thievius Raccoonus; in Sly 2: Band of Thieves and Sly Cooper: Thieves in Time, there are 30 clues to be found in each hub world.

Sly Cooper and the Thievius Raccoonus
Clues are found in jobs within the first four episodes. Every job that has clues will have 20-40 bottles. The special abilities contains within each safe are contextualized as missing pages from the Thievius Raccoonus and appear as torn, yellowing pages when collected.

One vault in each world, except "The Cold Heart of Hate," has a reward that allows Sly to see where clues are located, through the help of his binocucom. When viewed in the binocucom, clues are marked with green reticles.

Sly 2: Band of Thieves
There are always 30 clue bottles located in the episode hubs, and are not found indoors. The vault is hidden somewhere on the map, whether in a job area or another secret area solely for the safe. Unlike in Sly Cooper and the Thievius Raccoonus, the presence of the abilities within the safes is not explained, and appear as an icon representing the action that the ability contains.

In addition, this game introduces ThiefNet, meaning clue bottles are no longer the only way to obtain new abilities. Also, clues can no longer be located through help of the binocucom.

In The Sly Collection, the noise of the clue bottles can be heard from a greater distance than in the PlayStation 2 version.

Sly Cooper: Thieves in Time
Absent from Sly 3: Honor Among Thieves, clue bottles have returned in the fourth game. There are 30 of them in each hub world, and the vault unlocked in each world via the bottles contains a permanent upgrade to every playable character.

One of the vault prizes, Clockwerk's Eye, allows the player to see where clues are on the map. It is obtained from the vault in "Clan of the Cave Raccoon."

Unlike in Sly 2, Breaking bottles now creates a quieter sound.

Bentley's Hackpack
The Clue Bottle can be found as a prize. For more info, see this page.

Appearance
In the games thus far, clue bottles are green, with a yellow question mark label and a piece of paper sticking out the top. In each game, they also "dance" on the ground, but in Sly 2 and Thieves in Time, they make a clinking sound so the player can locate them easier.

Trivia

 * Bentley introduces the clues exactly the same way in Sly Cooper and the Thievius Raccoonus and Sly 2: Band of Thieves.
 * In Sly 2, the easiest way to find a clue bottle is to turn off the music in the Options menu, and then listen for a clinking noise. This is also a viable strategy in Sly Cooper: Thieves in Time, although the noise is quieter than in Sly 2.
 * In Sly 2 and Thieves in Time, when collecting a bottle, it will shatter, sometimes alerting nearby guards to your presence. However, in Sly Cooper and the Thievius Raccoonus, shattering clues will not alert guards.
 * Sly 3 is the only game in the series not to feature clue bottles in any form (except for within the animated short Goodbye My Sweet).
 * In the Sly 2 demo for Sly 2, the clue bottles are silent.