The most important check is that the entire exhaust system is completely gas tight. If it is, then the fumes must be getting drawn in from the tailpipe via the cabin vent one-way flaps that are situated at the lower sides of the load area behind the bumper side extensions. If either of the flaps are broken, then fumes could be drawn in that way. If your Zafira is an older model it will also be subject to the Zafira heater resistor recall.