have you USED pierce magic, pierce shield on a smarmy lich and did it work???
No, I'm taking your word that it doesn't.
If it doesn't work, then it's cheese (maybe built-in), since relatively to the spell system it should work.
Or maybe, they just have a Spell Shield active!
Which, actually is altered by many mods (all those that use Detectable Spells)
I've discovered that the version used by Detectable Spells is ABSOLUTELY BUGGY, (wrong portrait icon, and mostly, it has unlimited charges!!!)
EDIT: must be a ToB bug. Unpatchable.
Second, Littiz how come the thief's ability detect illusion work on an invisible char protected with spell immunity: div.?
Well, first, it's hardcoded, second it should work anyway since it's "thievish smartness", not magic
Technically if you changed the flags of True Sight and made it schoolless, it should bypass SI: Divination as well.
If I'm not mistaken, Non-Detection works only with nonmagical invisibility (from stealth). True Sight will kill magical invisibility (from spells or items) regardless of the cloak.
With an exception: invisibility gained from the Staff of Magi. Last time I checked at least
With both the cloak and the staff, you're really undetectable. A bug, maybe.
So: thieves, and mages with that staff, have a good reason to use the cloak.