The Thief Meter is a representation of the health of the current player character and when applicable, boss or ally. The Thief Meter consists of the character's face symbol and a large health meter; the Gadget Meter takes up the remaining space below the health meter.
The remaining-health section usually matches the main color of the character's face symbol. As damage increases, the healthy section decreases in size from right to left. Depending on the character, the damaged part of the health meter is colored either red or a darker shade of the health color.
Entering the safe house fully replenishes the Thief Meter, but it can also be partly replenished by collecting health, which can be obtained by smashing objects or defeating enemies, similar to coins.
Sly 2: Band of Thieves and Sly 3: Honor Among Thieves
When the player has not been seen by any enemies, the Thief Meter remains static and only changes when health is lost (where it pulsates red once) or health is acquired.
When the player is in close proximity to an enemy or performing a sneaky action (such as interacting with stealth aura next to a ledge), the Thief Meter will emit yellow rings, and the background music will typically transition to a staccato tune.
When the player has been discovered by an enemy or is under attack, the face symbol in the Thief Meter will begin to continuously pulsate red (and the music will become more fast-paced), and will continue to do so until the player has escaped or all pursuing enemies have been defeated. Once that occurs, the Thief Meter and the background music will return to normal.
Sly Cooper: Thieves in Time
Thief Meters have been modified from their original functionality and appearances in Sly 2 and Sly 3. They are now slightly transparent so they do not completely block the background. The character face symbol's eyes consistently flash white, and other accessories have special animations as well (the Thief Costume's earrings shine and El Jefe's cigar smokes).
When the player is in close proximity to an enemy, the Thief Meter no longer shows any indication of it (the music no longer changes, as well). However, it still emits rings (red instead of yellow) when the player is doing something sneaky.
When the player has been discovered by an enemy or is under attack, the Thief Meter now shows flashing red and blue lights behind the meter, instead of the face symbol pulsating red consistently. The music does not change. Taking any amount of damage, whether inside or outside of battle, will cause the character face symbol to pulsate red once and then return to normal.