I've had this problem several times usually after listening to music for a while (CD Changer) with the engine off. It also usually means I need to reset the date/time. Running version 27.1 SW. It's been to dealer a few times but no problem detected. It does make reference to resetting date etc in the manual when the battery runs low.

__________________

Fastbuck
2009 F01 7 Series 3.0D SE Saphire Black/ Oyster
2008 E83 X3 2.0D SE Montego Blue/ Black
2007 E70 X5 3.0D SE Space Grey/ Beige - SOLD
2006 Land Rover Freelander Sport TD4 Zambezi Silver/Black - SOLD
2005 W203 C200K Sports Coupe Jasper Blue/Artico Black - SOLD
2004 W211 E320 CDI Avantgarde Brilliant Silver/Alpaca Grey - SOLD
2001 E39 530i SE Crystal Green/ Beige - SOLD