very weird. I had a 2011 535 with no comfort access. It would sometimes be parked for a week or more unlocked and never ever got that battery warning.
2008 Mercedes-Benz CLS550 Iridium Silver/Black Leather -- P2/321/881 (leasetraded 1/10)
2002 BMW 325i --Topaz Blue/Natural Brown Leather --SP/PP/HK/CWP/Bi-xenon (Sold 9/06)