They are always up provided that you scouted the other players' homeworlds, and that the artifact on that planet has actually been found by anyone.
Once an artifact is found, all empires see the symbol under the planet and know one is there - but only the finder knows what it is.
Once you scout another player's capital, you will see the symbol for it after you leave. If the player changes the capital to another planet, it will not automatically update, you will have to discover the new capital. It's similar to fog of war where it shows you what you discovered, just doesn't update it. So in <insert RTS here> you might find some enemy building and when you leave it still shows you through the fog of war that the building was there, but the next time you have line of sight it may not be there anymore.
