Originally posted by Xyx:
<STRONG> (I guess they hear or smell him, or have some sixth sense that warns them).</STRONG>
As far as I know, faint vibration of air, somewhat like that around hot objects, can be seen around invisible creatures. Someone intelligent (a mage, usually) can probably detect this. Most people, seeing such shimmering, wouldn't notice anything or would just think their eyes are deceiving them. But a mage, having experience with invisibility users, would recognize that effect and use a True Sight or similar to be sure.
Ok, that's the technical explanation. The more obvious one is that Bioware is trying to prevent the party from killing the strong enemy mage before any battle is fought.
In addition to the Cloak of Non-Detection, Boots of Speed are good when using invisibility. In Planar Sphere, I killed the mage of the Halfling Warriors (I was at low level then, it was a tough fight) by casting invisibility on Anomen, who ran quickly near the mage and whacked him with Slay Living before he could do anything.