After few raids in which I collected shamelessly artifacts from neighbours and getting few in fair manner, I started receiving questers to my tavern. Each time there was message about one of my dwarves revealing presence of artifact in question and after few months there were direct assaults.
But then a spearman come, which didn't inquire random dwarves. What this spearman had done, was going straight to the mayor and making - adequatly - spearheaded question:
https://imgur.com/RgXBcwFBeing an avid epic reader I tried to refuse, but I couldnot bring myself to it. Since i didn't recall all artifacts names, I thought he might try to recover one of the stolen artifacts. Moreover, he charmed me with his attitude and "epic journey" backstory.
But then it turned out that it was one of my artifacts, made in fort. I have expelled some of my dwarves after cleaning bodies from last siege including few legendary craftdwarves, so there was slight chance that he acts as a representative. But I decided to browse legends.
Turns out this "hero" is mercenary, who instead of participating in battles became a notorious thief and hound to some human schemer making artifact claims out of thin air.
I have all my artifacts in museum, now locked behind the door. I am waiting for things to escalate, but for now he doesn't mind having know possibility of physicaly acquiring his "price".