All runes so far are in pairs or more, except the red rune. It seems the second red rune is nowhere to be found. But it lies behind a cave entrance with no indication. Here on the screenshot is the entrance to the second red rune. Make sure someone is over the first one back on the way and move one of the heroes exactly where is shown(the tree green arrows point) on the screenshot. The hero will appear exactly over the second red rune and the secret door will open. There is enormous treasure there no doubt.