Fumes in A320 series are often down to oil in the hp air ducting. This can come from the engines, but the APU is often the source. When shutting down the APU, the operator should wait for the air intake flap to be fully closed before switching the batteries off. This occurs at about 7%N. If they are too quick, then the oil vent discharge can be sucked back into the intake leaving oil residue in the system.