Question about rogue stealth

[Ripperzipper]

how does this work? I find even when stealthed with a reasonable Cunning score as soon as I step into a dungeon like 45 enemies are already turning and looking my way drooling. (Ok so maybe not 45 but I was just playing L4D and stealth in this game reminds me of running into a room full of zombies and they all turn and gasp)