Alex Turner says that Leaving is their last song that is the "original" arctics sound - songs about going out to clubs and havin drunken nights, and he wrote it towards the end of the time he wrote the album, so it is the last of the old album sound.
maybe this means it wont be on the new album as it may not fit in with the sound of the album - also makes me wonder what they are going to write and sing about now...