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Mystery solved (yes, I am like a pit bull when I can't find something). It's not a cover, it's just the top of the fuel filter (something that shows up in NO MB diagrams). Very odd...
The part number for the 2010-12 GLK is 2214701790 - it may be for the PZEV vehicles only (are they all PZEV, or just those that came through the California area? I believe both of mine did).
Here are a couple pictures of it (a Beck Arnley part available from Rockauto for about half the price of the MB part, though without the fuel pressure sensor). I hope this post helps others looking for this info in the future...
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
