I think Burnerman had it. I haven't had experience of alternator fault causing it, but try that, if not alternator, then take out all the fuses and see if the drain stops
If so, put them back one at a time and see when it starts again.
That will tell you what circuit is causing the drain
Since it occurs with engine off, it is probably (unless a wiring fault) a permant live, for example radio, brake lights, interior lights, alarm, horn, starter motor thick cable.
If I were you I would also start asking around for a recommended local auto-electrician in case you can't quickly trace and fix it yourself.