As far as I am concerned the "mystery" shield, so far, is non-existent. There has been some comment that there is supposed to be a shield in that position but no one, as yet, has backed up their comment with proof. Produce a photo or a part number please. There appear to several of us reporting this problem. My car has been with this condition since I purchased it new off the dealer'* lot the last day of April of 2005.
I will be going to the dealer soon for a regular oil/filter change. I plan to visit the parts department and check the microfiche for this part. I would be happy to purchase a new shield from GM if this would stop this unsettling condition from happening.