We had a friend with an Aero arrive here a couple of years back and it turned out that a seemingly good battery was going flat overnight however if he operated the inertia kill switch, thereby disconnecting the battery, it would start without problems the next morning. We went to the local auto electrician and discovered two things: 1) there was a constant drain on the battery that was enough to flatten any battery overnight, and 2) the battery was a little down on power and needed replacing.
It was too difficult to identify the source of the drain but when the battery was replaced the current drain dropped back to virtually normal for the various small items that had to remain live to some degree, central locking, radio, etc.
So it seems that if the battery is not fully up to the correct voltage some item in the car was not fully disconnecting when the ignition was turn off. With a fully charged battery the item turned off correctly.
