Started a new Fortress just to test out some things -
Created my normal Four-Person Squads for local work - defending the Fortress, hunting down giant animals, so on.
Created a Ten-Person Artifact Squad and sent them to a Tower. They were able to sneak in and steal four books. But on return two of the soldiers, Mercs, were no longer part of the Squad. And I put them back in and sent the Squad to the same Tower. They were able to sneak in AGAIN And steal four books - copies of the same FOUR books. I guess the necromancers just like to make copies? And the Mercs, once again, removed themselves from the Squad. So I assigned local Dwarfs to the Artifact Squad and put the Mercs into one of the Four-Person Squads. The Artifact Squad was sent out, this time detected, KILLED at least two necromancers and came back empty handed (with a few missing teeth and fingers). All Ten were still in the Squad.
Most of the books disappeared as visitors claimed a few and one seems to have been claimed by the Mayor(?).
I have a locally made Artifact on display in the library but plan to move it to a proper Museum later. Other Artifacts made in the Fortress were claimed by their creators as family heirlooms. Did have some problems putting books on display but I think that was because some were claimed or already taken by visitors. The message seems to say the item was lost or destroyed.
I have a ton of visitors but have only allowed a few performers to join but take any hired swords who ask to join. Only had one were-beast attack so far and no vampires (or they learned not to announce themselves).
Raided a Elven Retreat and was able to grab two books without being seen.
Also explored a few Elven ruins on the off-chance they could have something. Even visited the ruins of my first Fortress in this save that lasted less then a year. I KNOW it once had three Artifacts but none were listed and none were found.