Water ingress through roof bar fixing holes?

Mk2 08 Fabia: 

A couple of days ago, it snowed.  Then, after it thawed, I noticed the upright part of the near side rear seat was a bit wet.  Couldn't see anything obvious.  Checked window was fully up.  Yesterday it rained a lot.  Today the same seat was much wetter.  Roof lining dry.  Wet is too high to be coming in through the inside door panel.  Door seal looks okay.   However, I spotted a hole in the body, where the top of the door would close onto.  Seems it's where roof bars would fit into and when I pressed a finger up to that hole there was a drop of water.  Seems daft, but it's almost as if the body shell (or part therefore) has filled with rainwater and has now got to the point of seeping out of the hole, dripping onto the seat. 

If that's the case, how would water get in?  (No obvious gaps e.g. around aerial) And is there a way of draining it away?

Car is kept on drive, with an upward slope i.e. front wheels higher than the rear ones.