What's invisible cannot be seen. What's unseen can be seen, but is or was not seen. It's possible to see things that are invisible with tools, like heat sensors, microscopes or something like that. 'Invisible to the naked eye' would imply such a scenario. 'It remains unseen...' is a phrase often used to describe when something has yet to be tested. It remains unseen as to whether or not recent changes to the school system are having the effect that the education board intended. These changes could be seen possibly in the future.