Yes, one time when I had mine detailed they told me they found the water in there when they vacuumed the trunk. Same thing, the water was not in the trunk itself, but in the lid. I have not had the problem since and attributed it to the fact that they were using a water under high velocity and pressure. I did not give it any more thought until I read this. Perhaps you should check with the dealer and see if others have had the problem.
It appears that it eventually drained out from the rain we got, but I washed the car this evening and it did the same thing. If I pull on the lower edge of the plate recess panel I can get it to dain faster - otherwise it just drips out until it empties.
Fortunately it seems easily reproduced. Only place I can think that it would be getting in is at the upper seam, which runs the length of the trunk lid, where the panel meets the trunk lid.
Yup! The diagram shows the license plate recessed subpanel as a separate attachment to the trunk lid. I wonder how many screws holds it on and is there a rubber gasket???
Definitely a collusion or damage to the rear portion of the Trunk Lid could cause a leakage in that subpanel area. Maybe just slamming down the trunk lid really hard could cause the panel to bend enough to let water get throuigh?