I had Serpent's Bite generated in UD:3, guarded by an extremely experienced green hag.
I shot her with an arrow, missed, she moved several squares in my direction, I teleported to the artifact and used it to slay her with ease.
Something is not right here, artifact guardians should pick them up when the move from their original spot.
Orbs and RotHK should of course be exceptions to this rule, as should TotRR, at least for Khelavaster.
No problem if a greedy farmer jumps over and picks it up, he might have lost his scythe somewhere and it's harvest time already so another pole arm should do just fine ;)
A PC at this stage of the game has already done some amazing things on the way, might as well just teleport away everyone who could pick up the trident, just to be certain nobody else can take it.