When the fuel pump went on my 2005 Passat that's where the breakdown guy went first. Back seat out, and underneath on the drivers side was the pump, which obviously sits in the tank. I'm not 100% but I think the fuel tank is right under the seat, where the fuel pump is located. Maybe the sealing ring has gone/perished. Might be wise not to fill the tank too high until sorted to minimise the risk of further leakage.