From Halopedia, the Halo wiki
The radar jammer is an equipment item in Halo 3 that scrambles nearby players' motion trackers by creating a multitude of false dots on the radar of everyone within range. While it can create chaos for enemies, it also has the same effect on teammates. In campaign mode, it is uncommon for one to be dropped by Brutes, but is often used by Brute Stalkers to make them even harder to be found. It is no longer available in matchmaking as it, along with the flare, had physics properties that allowed players to exploit the game unfairly.
The radar jammer is similar in appearance to the power drain and flare, except for a red glow instead of blue or yellow one. Its symbol in the heads-up display is a dot with curved waves emitting from it.
While under the effect of the radar jammer, it is possible to distinguish between the 'false enemies' and the real enemies that are displayed on your radar. The dots on the radar generated by the jammer almost never stop moving, whereas real enemies tend to move more carefully and stop at points. The fake blips also seem to move faster and more "fluidly" than a real player. An activated radar jammer can be destroyed by weapons fire.
List of appearances