There is a trophy in each of the Uncharted games called 'Relic Finder'. To earn it you have to locate the 'Strange Relic'. I don't think these relics are mentioned in the stories at all, and I have found the one in #2 and it just shows up like a normal treasure you find. I haven't noticed anything that it changes yet.
So, what is the significance of the "Strange Relics" in each game? and why are they different from the other treasures you collect?
In answers please use spoiler tags if the answer relates to the story.