From The TimeSplitters Wiki

Invisability is a power-up that makes the user invisible for a limited amount of time. This is especially uself against human controlled opponents but AI Bots tend to know where you are in TimeSplitters 2. AI Bots will usually just run right past the user in TimeSplitters: Future Perfect, but will open fire if they are fired upon. While the name may make one think it is complete invisibility, this is not true. Instead it will make the environment appear wavy and warped like in the user's location on screen.
These powerups are distinguishable as they are large, yellow spheres with what apears to be some kind of cloak inside them.