Maybe there's a drain plug on that side that’s clogged and not draining properly. Or maybe there's a plug that should be there and isn't. Just a wild suggestion. Let the dealer get it up on a lift and see what the underside looks like. I'd also try and get it there in the rain at its worst. Don't wait for the weather to improve or for the car to dry out, then they'll say they can't duplicate the problem.