Master Chief:
FPS_Chris:im thinking sometime between harvest invasion [2525] and reach [2550]...it was definitly a human held planet in between these two
Well there was some cov. structure.
More likely those were either human or Forerunner structures. To the best of our knowledge, no human force has ever made an incursion on a Covenant world. That would have been the Master Chief's mission aboard the Pillar of Autumn before Reach was attacked: to infiltrate Covenant space, kidnap a Covenant leader, and ransom him
However, we do know the Covenant are scouring the galaxy for Forerunner ruins and artifacts. The Prophet of Truth, during Halo 2, mentions that humans have "blocked access" to sites sacred to the Covenant, thus leading to hostilities. In Halo Wars we may get to see the genesis of this conflict; UNSC forces defending human colonies from Covenant attacks, while the Covenant are trying to secure artifacts and sites they believe to be imperative for their "great journey".
As for the rings themselves-- I doubt we'll see one, as it would be hard to explain why the discovery of Installation 04 is such a surprise in Halo 1 if human forces had seen one as much as 25 years prior.