Have your battery tested first. You may just have a dying one that does not hold a charge as well anymore.

It's not always the case of course but it is the easiest to check. I've gone on an exhaustive search for battery drain circuit by circuit, found nothing, and have a new battery fix the problem lol! Like in your case, it would work fine most of the time but would go flat at random.

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04 X5 3.0i auto
03 X5 4.6is