From The TimeSplitters Wiki

Cloak or Invisibility as it is sometimes called, is a glowing yellow sphere that contains a black veil. It temporarily turns the player almost invisible, leaving just a shimmering, translucent outline. With this power-up, opposition players will find it difficult or even impossible to spot the user. In TS2, AI seemed to be fooled by the effect for only a brief amount of time. TSFP's bots were slightly easier to trick. They usually will not be able to see the player until they are fired upon.

Trivia

• In TS2, the Scourge Splitter's purple flames are still visible when he is cloaked.