I noticed this on mine earlier as one of my LED number plate lights was dim and I thought it was dying.
In the process of lifting the boot lid, I heard and felt a big swoosh of water and it was obvious the boot lid had a lot of water in it.
Having just been out to investigate, the boot lid is empty and I can see a plastic mesh which lines a small slit at the very bottom of the boot lid so this may well be a normal draining situation.
I took the boot lid lining off and it is all bone dry so I suspect the boot has some sort of second skin or deliberate draining mechanism through it.
Having taken out and re-seated the number plate light, and ensure both are clean, they work fine again.
Most odd.