Due the the volume of water/rain a lot of us had in the last few days, I would suggest lifting & lowering your tailgate to see if you can hear any water inside the tailgate cavity.
I removed the two maintenance covers for the rear light clusters and there is/was water in this area, just now when I checked.
I took my car the the dealer yesterday, and we found two small plastic bungs in the drain holes (bottom left & right of the tailgate) they removed these, most of the water had drained on my way home.
I would suggest you check to see if these bungs are there in your car, if they are, don’t remove them but take the car to your dealer & ask them why they are there.
They are small and possibly get forgotten during the pdi.
The real question is why water is there in the first place, I’ll be going to my dealer on Monday to ask!
I have pictures if any one wants to see them.