[Kaleban]

Well, I have the Cloak of Non-Detection on my Mage-types usually, taking advantage of its anti-detection capability.

True Sight has NEVER dispelled any invisibility for a wearer of the Cloak. The reason you may be having problems with Thief-types is that they must have Shadows to hide in. They have to check each round to remain in Shadows. The Cloak does nothing for a Thief in broad daylight.

Its also possible that if you set your thief to backstab, that he missed with a high initiative, which means he would pop into visibility and be targetable, same as if a mage cast a spell while invisible. The Cloak only protects from "hostile" detection attempts, any hostile action on behalf of the wearer (including failing a Hide in Shadows roll) negates stealth/invisibility.

Hope this helps!