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. Against humans however, the invisibility powerup turns from a full blown advantage to simply a niusance to the opponent. the combination of the cloak shimmer and the radar dot means finding a cloacked player is simply a matter of looking in his general direction and firing madly.

Trivia

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