I had a similar problem with my very old pajero. The battery would go flat after a while and i'd have to jump start it. then Tt'd go for days and days then nothing.
I took it to an auto elecrician who said there was nothing wrong with the battery and the alternator worked fine. He tested it (the battery) for current draw when stopped which would show a short curcuit from a loose connection or similar ( i watched him do it) and nothing showed.
Consequently i was forced to buy a new battery. Whilst replacing the battery i found that one of the additional wires running off the battery (to power the radio) had an almost broken connection. I replaced that connection (actually i taped it up with duct tape) and everything was back to normal.
I assume the current draw was so small that it couldnt be picked up on his meter, Either that or it was only shorting when it was in a certain position.