To me sounds like it could be one of many things. The air-con seats if equipped could be acting up, the heated seat could be acting up. Or my best guess is it is the power seat function somehow malfunctioning causing it to drain the battery. When using moving around in the power seat it uses quite a bit of electricity, and can easily drain the battery if malfunctioning. Best bet although I hate to say it is the stealer. Unless anyone else can think of somewhere or can do it themselves.
2005 A8: Power Boot, Cold Weather Package, Summer Package, Blue-tooth Package, Satellite Radio, Keyless entry + Start, Air-Con Seats, Upgraded Leather Trim, 2011 Navteq Maps, and Escort 9500i.