Water in Battery Compart

After recent rainstorms here in Alabama, I detailed my 2001 E46 Sport Wagon. I discovered about 1/4 inch of water in the battery compartment! Anyone has had this problem? There is a small white rubber hose hanging, connected to what I guess is the outside of the fender. Any ideas how to find the leak? Thanks y'all.

With a helper, one person watches the inside while the other one sprays water from the outside at the hatch seals, window seals and the gas filler area. Another possibility is that a "body plug" may have fallen out; there is one roughly bumper height on the back side of the box. If this plug is missing, water can spray into the battery box when driving in the rain.

A third possibility is that rear window washer fluid has leaked into the battery box. The tank is above the battery behind a panel. Most battery vent tubes are black, so the white hose you found may be a disconnected hose in the washer system.