From what you say it sounds as tho' the water is coming up through the floor in the area of the bung. especially as you aren't getting any water traces from anywhere else. Is the bung cracked, holed, damaged at all? You could take the bung out , blob a bit of silicon sealer around the hole and replace the bung. Leave to set. Water can get anywhere as it tracks along any ridge, weld or whatever. It will get thrown up by the wheels and the wind as you drive along will blow it up, down, in, out - anywhere really. Drive up on to a pair of ramps, get a pal to hose down the underneath while you sit in the car and watch for water ingress. I have had hours, nay days of fun trying to find leaks in Land Rovers products, but then the saying was 'the holes are for letting water out as much as letting it in' !