(1) such a civilisation would possess capabilities beyond ours to convert energy: it is possible they would use gravity. Either way I would expect zero emissions. (2) such a civilisation would not want to entice the curious and less intelligent to come knocking. (3) they might have already shed their mortal coil. But the question strikes me of being one more deep seated in the human condition of shouldering the burden of responsibility, and of reconciling hope and fear. Hope that we are not alone, and fear that we are the last civilisation, or that the universe is just not into humanity.