2006 BMW X3

My 2006 X3 runs like new even though it has 140K miles. But a about 1 1/2
yrs ago, the battery began discharging, at random times, and never at the
dealership service dept over the two weeks it was there.  Even with a brand
new battery. I even had the new battery replaced after a year thinking that
was the culprit.  Read a blog and learned this could happen from security
system, so made sure car is locked so the security system won’t work as
hard, which would make sense why it started every day at dealership.  
Unfortunately, after two weeks of thinking I was a genius, I got in my car and
battery was dead,again.  Alternator, starter, all good. Lights on auto and I
even unplug any phone chargers.  Any ideas?  Nothing registers on a scan.