Progressing through the campaign and completing custom missions rewards you with unique content: Tea and Cupcakes Mission : Complete 50 Custom Missions to unlock this secret mission.

On the mission (Level 4), walk to the southeast corner of the vault room. Crouch and look at the wall. Spam the Action button (X) . Your character will clip through the wall, revealing a hidden room containing a Golden M9 pistol . It holds 20 rounds and has a "One-Hit-Kill" property against unarmored enemies.