This is a very similar problem to one I have with my Subaru VX. I did a electrical draw test on all the fuses and found that the ABS system (Fuse SBF8) is drawing power all the time, the current reading was 260 milliamps. It flattened the battery overnight. I now have to track the fault in the wiring that is causing this. Going to try pulling the fuse when the car is not in use so no power is drawn.
Peter