I had similar problem couple of times.... Both times when I had the car sitting unlocked for extended period of times (like loading it up for long trip).

First time the dealer said it was a bad battery, replaced it. Second time they said it was a bad terminal connection, and they tighten it up.

I believe it is all the electronics in the car, and not strong enough battery. When you unlock the car, the computer starts booting up, all electronics get in stand-by mode, etc. No wonder.... It is also possible that there is a sensor, which detects discharged battery and prevents engine startup, which is miscalibrated...

I thinking of getting a car battery charger and carry it with me. Something like this : http://cgi.ebay.com/JUMP-START-CAR-B...3A1%7C294%3A50

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His : 2013 X5 3.5d Deep Sea Blue/Black/Aluminum with ALL packages, 20" 214"s, running boards, Adaptive Drive, Spare Tire, BMW Apps
On Order : 2016 X3 3.5 MSport Silver/Mocha with DHP, CW, Prem, Tech, DA, Lighting

Hers : 2015 328xi MSport Silver/Red

Prior:
2008 X5 4.8
2011 X5 3.5d
2010 335 xDrive
2013 335 xDrive