Thieves and invisibility

[Guest Guest]

How does the spawning of invisible thieves work exactly? I'm having an annoying occurance in my game: a battle starts (a random city encounter or the battle in Mae'Var's guild, basically whenever some hostile thieves start invisibile), I cast Glitterdust, no thieves are revealed but a microsecond later I get backstabbed from a position that was within the area of effect of the spell. This becomes aggravating very fast when you're playing a character with high AC on Insane and a single backstab will trounce you.

[DavidW]

They spawn already-invisible. (Already hidden in shadows, is the idea). I'm not sure why Glitterdust doesn't show them up; will chase, when I get a chance.