Traced my issues to the audio head unit. Noticed that aftermarket GPS that plugs in the back of the head unit did not turn off sometimes - ie GPS remained powered up long after ignition is off. Never an issue if I turn off the head unit manually before exit the car. You might try this for a while and see if cures your issue - at least will isolate the problem to the audio head unit. I can leave car standing for 2-3 weeks now with no issue.
VW Touareg V6 TDI 4Motion
X3 3.0d 2006 (MY07) being sold