S.T.A.L.K.E.R.: Clear Sky

They do, probably emissions but they do respawn

I originally thought they respawned as well, but what is actually happening, is that you are upgrading your scanner/tracker thing. The better your scanner, the more artifacts it can detect. So when we start out with the lowest level scanner it would not detect all the artifacts in an area, but when we come back to the same area with a better scanner, it will show them. This gives the impression that they respawn, but I don't think they actually do.

They do respawn. Not every location respawns from what i can tell but I keep going back to the first jellyfish you find later the very start of the game and can keep finding a jellyfish at that same spot.

Areas like that hellrift type deal you find near the freedom base *its actually south east of the freedom base I think* that area wont respawn as far as i know.