Good luck….water ingress is one the hardest things to diagnose on a car as where the water is pooling may not be the point of leak. First thing id check is the tail door seal and make sure you have good closure against the frame and seal. You can do this by closing the tail door with a piece of paper in there to see if it traps it. Look to make sure its aligned correctly and visually check the seal itself. Other than that it could be literally coming in anywhere
Edit…no theres generally no drain holes in the boot floor…however if you cant find the leak it may be worthwhile temporarily making one!