Successfully complete Campaign mode on the Insane difficulty setting to unlock a secret Gamerpic, which has the Red Gear's Symbol behind Marcus. Successfully complete Campaign mode on the Casual or Hardcore difficulty setting to unlock Insane difficulty setting.

Note: You can only tag people and if you switch weapons you will exit the glitch. The ammo counter should read zero but you should be acting as if you are holding a grenade. Then, press RB(5) or RB(10), depending on the game. Take out your Lancer and do an Active Reload then immediately switch to your grenades and press B when switching. First, get either smoke or frag grenades.

This trick can be done on any online level.