would have to be something left on to create amps drainage. You may have forgotten to turn off the boot light or interior light. someone from your family would have accessed the vehicle and left one of those lights on by accident, or turned it on and realised to late- turning it off hours later.
or your vehicles battery was 4 years old and the last time you used it- you may have left the headlights on even for just 10 minutes, with the engine off. which requires a jump-start