f7f18077_r_PlayStation_3___GameRankings__Unlockable
[How to Unlock] Stealth Item [Unlockable] 
Complete a single-player session without instigating a single alert (caution is okay, but not alert). You can do this on any difficulty
Metal Gear Solid 4: Guns of the Patriots Cheats, Codes, and Secrets for PlayStation 3 - GameRankings