There have been many posts on this and other forums about BMW battery issues whereby the battery wasn't the problem, but went flat due to the vehicle having too high a current draw. The vehicle is designed to go into sleep mode, and then it draws very little power. If something stops it going into sleep mode (like a tiny light flashing) then the current draw can flatten the battery in a few days. That sounds like what happened, IMO.

If you got a new TCU under a goodwill policy, then that is great. Your battery is 4-5 years old, and that is about how long a battery lasts. This likely just pushed it over the edge, but it was either due or very close to being due to be replaced in any case.

All of this is speculation, not having been party to the diagnosis, but it fits.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue