Weird Battery Behaviour - 6 Volts

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G4 Impreza with 35k on clock - this morning Battery completely dead (I think we left ignition in Acc mode over the weekend)  ...this morning no instrument lights, nothing.  Checked voltage and it was only 6 volts - I assumed battery  is stuffed.  I tried little jump starter but it wouldn't turn it over (which is weird as it can start my Forester) anyway I had to move the car so I could get the Forester out of the garage and so I jumped it from the Forester with engine running.  Then I drove the Impreza about 200 metres let it run for 5 minutes  and turned it off. Battery now reading 12 volts - restarted ok.  

I have put it on charger but I can't for the life of me figure what is going on.