[M-A8]

OK.
Now we need to find where the water comes from
Shame you didn't what I suggested earlier, leave the boot open, stick the hose on the roof so the water runs down the rear screen but no excessively so it dont go straight into boot of course, or something like that, and watch for any running water there where you found it today.

Is it possible guys that water gets somehow behind the rear screen and runs down straight into the boot but somewhere on the side panel side???

How about the boot seal, I assume you looked at this properly and if that would be the seal then you will find water inside the actual boot as well.

Condensation on the positive wire would indicate that water is getting through the top???

Now, you have two unit there, I assume there is no faults in the VCDS on those two units??
If they would be slightly water damaged they could drain the battery.

Above the batter where the (-) is there is I believe battery management module, if condensation gets as high as the positive wire then it would get there as well. If there is any water damage to it that could drain the battery. Well, any water damaged module could be responsible for battery drain.

Did you try to lift the rear seat and see if there is any sign of water or dampness???

How about the seal around the fuel cap, which is just about there, but from the inside??? This would kind make sense as its VERY close to where you found water today. This actually could be it.