Water leaks can be a real pain. Water can 'track' from a completely different area.
If it was me, get the help of a friend and whilst one of you sits in the car, doors shut of course, get the other to put a hospipe on the rear end and watch for a minute water trickle. It may just be a glistening few drops of water. Even in good light, the use of a torch may help. It may take some time to show but your patience will pay off