Battery Totally Dead?!

[triumph2.5pi]

Hi all
My '00 2.8 Sport's battery has just inexplicably and suddenly died. No prior indication of a fault (I've had the car for 2 years or so and all good), it just completely drained from one day (or maybe two or three days) to the next. I'm a complete layman, but I do have a multimeter(!) and it's registering 0.7v! I've taken the battery out and currently have it on an overnight trickle charge.
I do have trusted local Audi specialist I can take it to, but I just wondered before I do that, if there might be a simple reason that I can maybe fix myself why this might have happened? No, I didn't leave the lights on(!), or anything else that I can think of...
Any suggestions at all?
Thank you! :-)