On Mount Chiliad there are petroglyphs relating to the Mount Chiliad Mystery that can be found. The first and most obvious one is a red drawing of a UFO at the peak under the viewing platform. On the platform above the glyph can be read the message "COME BACK WHEN YOUR STORY IS COMPLETE."[1] Which means when you get 100% on story mode. The rest can be found on the slope of the mountain. When viewing the mountain from the west in a helicopter you can align the Mount Chiliad mural with the actual mountain to get the locations of the petroglyphs. [2]
The drawings in relation to the Mount Chiliad mural.