Today I found the battery of my '97 XK8 to be totally flat; Seems to have been caused by ny leaving the driver's door open for some 48 hours. Wonder what electronics could have consumed all my energy? The interiour lights are automatically switchd off after some 20 secs. The key was in the switch, but not switched on.

Anyone any thoughts?

 

Yes, this would be expected. The internal communications network will stay active and not allow several electronic modules to go into sleep mode with the door open.