If they stayed truly invisible, it was quite likely that well-known Bioware display issue where creatures do not appear. If it were a haze/semi-transparent visage, it might have been Improved Invis. A bit more information about what you have seen would help to understand the details of this issue.As for giants using stardusts in a dead magic zone, this is along the lines of undead using healing potions in their inventories. These items get generated automatically by the loot system and sometimes there are odd things that happen as a result. I'm fairly positive that the giants are not simply equipped with these stardusts by default.