Could it be that your front cap needs to be re-sealed? Possibly causing some water to trickle down into the bottom around the pin box? Then when you lower the 5er, the water would either run out around your pinbox, or into the floor of your basement. Just a guess, but if your 5er has been in the rain, that could be a reason for it...If not in the rain and your front cap is sealed well, I'd make sure you don't have any leaks in any plumbing in that area of your 5er...
ON EDIT: Notice you have a 2013, which would probably mean it's not the front cap seal, but again, one never knows...I'd make sure there are no "slight leaks" anywhere in the plumbing up front...Check seal around the front cap anyway..