Yeah that is a trap. When I was keen and younger and washed my ute, when you opened the doors to do the interior they always sloshed with water. Quick poke with a screwdriver and away it ran. When you pull the trims off sometimes the inside of the door is full of broken glass, blocking the holes.

My wb had some sort of putty blocking the drain holes in the rear beaver panel. Seemed to be done in the factory. Assume it was to stop dust getting in there? I pulled it out to let it drain.