Have they checked and verified it is not the "Radio drain" problem? I think there was a recall sometime back on the year you have. Try disabling the radio (take the fuse out) and see if it still drains the battery.

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2017 Tesla Model X
2014 Porsche Panamera-S e-Hybrid (sold)
2012 Porsche Cayenne
2001 X5 3.0 (sold)
2003 Carrera 911 Cabrio (sold)