A problem i find in the occasional modern rpg is that unique items are often too easy to find and are often done so by either random drops or in a chest on your way to a greater goal.
Another traditional method of acquiring them is through boss fights. Yes ok, fighting a particularly powerful foe should yield its rewards, but for those items, these legendary artifacts that have changed the course of that games history, they should be secretive, they should not be found simply as a drop to a boss you had to kill anyway.
Dragon Age has the same problem, some of the most powerful items there are found simply on your way to another objective, in obvious shining containers. What is the gratification in finding a beautifully crafted weapon, giving you an edge in combat simply as a prize for not being completely blind?
Skyrim had a good idea with locations of certain artifacts hidden in books. The problem was that it marked the location of the item on your map the moment you picked up the book and left nothing to chance, the book itself was also commonly found.
Playing Fable, i recall the most powerful sword required a certain alignment for the tomb that contained it to open, another was a sword-in-the-stone type where you could see men from the local village trying their strength attempting to pull it out, not to mention certain chests that required a particular amount of keys found throughout the world. That is what i would like to see, that there is some challenge involved in finding an artifact of significant power, be it trough optional side quests , cryptic requirements or deducing their location through obscure data. If you absolutely cant find it and still want it, ask google.
PS: I make a distinction between unique and legendary items. Legendaries are certainly unique but are distinct through power and historical relevance.