Has it rained recently? I am wondering whether or not it is simply where the drain holes from your sunroof exit! If it has rained recently and you have parked on a slight incline to the right and rear, this may well explain why the water is only appearing on the one side also.
It is just a thought that it may be something that simple as drainholes often loose their water from that area behind the rear wheel arch
Currently in scatz's garage: 2004 Subaru Legacy GT Spec B, a 2013 2.0 TDCi Titanium X Sport S-Max and a 2001 Honda NSR125R