The "Current Development" note 05/06/2017 mentions fortress artifact theft using essentially the same mechanics as current Kobold theft does, which is not going to work to get an artifact unless you deliberately build a very unsafe fortress (and even then, Keas are much more likely to be able to steal an artifact than anything on foot for a pure theft [i.e. no combat] path).
Are there plans to add fortress artifact theft mechanics that have better chances to succeed, and if so, would the target be for later in the current arc or for a future one?
I have to delve into suggestion-y territory to explain what I mean: Something along the line of tavern visitors who just happen to wander into the museum and drop the artifact into their pockets (basically the way vampires operate: hidden in plain sight). However, for this to be balanced, there would need to be some means by which the fortress could deal with those wanderers (guards who react to crimes [or at least theft of artifacts they're set to guard], restricted access areas [artifact thieves and vampires wouldn't respect them, but they would be respected by "legitimate" visitors, such as diplomats and petitioners], working [as in "activated by visitors"] pressure plates/traps or the like), short of just banning all visitors.