Not the same car, or to quite the same extent, but with both my MY10 D4 and RRC, I find voltage dropping visibly from the stabilised 12.6 within a few days - and both have newish dual batteries.
I've arranged a small north facing solar panel (30W from memory?) on the outside wall of the garage and just leave the alligator clips attached to the battery of whichever car is in there. No problem charging here in sunny Fremantle, and it didn't take many drive offs before I now remember (by leaving the bonnet up in the interim) to first disconnect. Sounds like a hassle but its a simple solution until the original problem is resolved - if ever.
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