Elite Rangers, Golden Elites
The "Golden Elites" or Elite Rangers (all named "Bob") are Elites with yellow armor and blue helmet visors. They appear in some levels and then quickly vanish into thin air if you don't kill them. It is unknown how difficulty level effects their appearances. You will get a medal for killing these called "Seek and Destroy." Nothing else is known about them. Here are some of the locations where you'll find these strange fellows:
Winter Contingency: On the Campaign level Winter Contingency, quickly make your way through the level and you will randomly encounter the "Elite Ranger". His armor is bright yellow and his helmet has a blue visor that shields his entire face. (He appears on both Legendary and Normal difficulty, but he seems to show up in different areas). Usually he appears during the part of the mission where you're driving Carter and Jorge around looking for the stranded marines. Kill him and you'll be rewarded with a special medal in your medal chest on Bungie.net. He will show up in your statistics as one of the types of enemies you've killed. In the stats section, it lists him as an Elite Ranger, and actually says his name is "Bob". -- Shamrock20
Nightfall: In the area where you meet up with the colonists fighting a pack of Covenant, this Elite appears -- and quickly dissapears -- amongst the buildings.
Tip of the Spear: On normal difficulty, when you arive at the Spire area with Jorge the Gold Ranger Elite (aka "Bob") will suddenly appear at the first "Bravo" capture point, in the game type Invasion, which would be on the left hand side of the multiplayer map: The Spire.
The Package: On the Package, once you enter the gate to Oni Sword Base (Rally Point Alpha), quickly head up the ramp and look to the left to spot the Elite. It's in the courtyard area where you meet up with Noble Team before heading into Sword Base.
The Pillar of Autumn: After the Bugger cave you'll come across two massive buildings in a valley. The second building you come to -- the one with both Data Pads -- has a a garage at one end (to the right when facing it). Inside the garage you'll find this Elite.