DraQ
Arcane
Then make noticing automatic the moment player tries to equip/use hotkeyed item, tries to hand it over (for example to a quest giver) or enters any relevant barter interface.First thing first, in order to have NPC thieves game needs a bit of asshole UI - player shouldn't be helpfully informed the moment he loses something from inventory, unless he willingly gives that item up.
The consequence of this would probably be a legion of casuals flooding the forums, claiming that the game is bugged, because their item disappeared. :D
You'd probably need the game to acknowledge it anyway in order to make tracking the thief down work - for example by letting you ask if anyone has seen an item matching description of your artifact.
So basically, getting your stuff stolen silently sets up a quest trigger that is tripped the next time you perform any of the relevant actions like described above.
The bottom line is don't let player know right after the fact, tell them after the thief has likely made his getaway and likely after they have already saved over so they can no longer render your crafty mechanics/procedural quest irrelevant by reloading.
The player should only notice theft right away if the thief botches the theft itself (meaning you start by effectively grabbing the guy by the hand and go from there) or snatches his prize but gets noticed in the process (we start with thief trying to make a getaway).
It might also be nice if killing the fucker outright could be considered excessive by the authorities or people witnessing it (depending on circumstances).
It always irked me how there was no meaningful reaction to, for example hacking that drunken fella in an inn to pieces in BG or cutting down that thief during the fair in Nashkel.
Anyway, the game should definitely react to player stuff like wealth (people asking for loans/donations, a distant relative pops up and asks you to fund his business...), fame (you get admirers and enemies, powerful people start to notice you...), gear (for every artifact, there are bound to be some people who want it bad, ranging from "Please sell it to me!" to assasination attemps) or any other assets (why confront the badass hero and risk getting killed when you can just rob his house? Living thieves aren't stupid. And you get a possible quest of tracking them down.)
There is a wealth of interesting quests that could be initiated this way, especially if their challenge rating is relative to what initiated them, making you think twice about what you can and cannot pull off.
The most ironic part here is that Bethesda is probably the closest company out there to actually trying to implement such stuff.Results in a nice feeling that you aren't the center of the universe and there are other/bigger things than you, something I'm sorely missing among the random-generated, level-scaled crap today.