Mobs have up to four senses they can use to detect your presence. Vanish/ga negates the Vision sense but you can still be detected by Sound (footsteps), Life (some detect a character with less than 40% HP) and Magic (casting a spell).

Birds, Giants, Humanoids, Imperial Army, Insects and Materials only use Vision and Sound to detect you, and they have very poor sound detection. Insects also use Life so you can sneak by if your HP is greater than 40%.