BOB

The BOB is a special type of enemy encountered in Halo: Reach. The BOBs usually take the form of yellow/gold-armored Sangheili Rangers. In some areas, BOBs take the form of a white-armored Elite with randomly generated armor, ranging from Minor to Field Marshall. These white-armored BOBs always dual-wield Plasma Rifles, regardless of rank.

Description
Killing one of the BOBs will reward the player the "Seek and Destroy" medal. However, the medal can only be viewed on Bungie.net. In the post-game carnage report, the BOB is referred to as a "light Elite vehicle". On Bungie.net, their description is simply "Wipe them out."

The name "BOB" is a reference to Bungie's Marathon video game series. In the series, all of the encountered human characters, besides the player, are known as "BOBs". "BOB" is an acronym for "Born on Board", a reference to the characters having been born onboard the titular colony ship.

BOBs are considered easter eggs, and thus have no canonical explanation in the Halo story.

Trivia

 * Each BOB has a set time limit before it vanishes. For some BOBs, this time frame begins as soon as it is alerted to the player's presence. For others, the countdown begins once the player enters a certain area, thus it is possible for the player to miss the BOB entirely if it is not spotted.