With it draining at random I would find out if the tcu is there and unplug it then see what happens. Its hard to find drains so being methodical is the only real method to find it.

__________________
Just BMW's - All cars and trucks was two pages long
1994 318is
1997 528i
2000 323i
2001 X5 3.0 auto
2005 X5 3.0 auto
2011 535xi auto
2013 X5 xdrive35 Turbo
More are at my website

Aftermarket E53 Radio Install