Why is my 2004 BMW xi have its passenger rear floor and floor mat wet from water?

[jpmarcucio@sbcglobalenet]

There's no sunroof in this vehicle. I don't see any watermarks anywhere that could indicate water coming in from the bottom of the door, the window, etc. The front carpet and under the front seat is bone dry. I looked under the vehicle and do not see any problems either. Where could the water be coming from? I thought maybe from the trunk but that's dry also. It's got me baffled. No spills that we can recall either. Any thoughts before I spend the money and bring it in to get serviced?