That is where the water is supposed to drain out. It can get in between the glass and the felt strip.

 

I am glad this topic got posted because this is happening to me now as well. Very annoying.

It happens when the car is parked with the front downhill, as reported above.

However my door isn't draining anymore. The water is just sloshing around inside while I drive. Right and left forward doors.

Now I am going to look for how to open the drains.

 

There should be drain holes in the bottom of the door. Sometimes they have a split rubber plug to allow water out but impede road noise in. The window is not water tight - water runs in around the belt line and out the door bottom.