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 the ONI: Sword Base level, 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 BOB's 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" may be 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
On page 384 of the Halo: Reach game guide there is a list of enemy point values and in this list is an enemy called a Daily Challenge Elite which has the same point value of a BOB, 200 on normal, so it is possible that these two are the same thus making that each BOB only appears on one day of the week.
 * 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.