Seems odd if you consider that the engine is behind you and you are moving forwards.
The reason is two-fold.
When you drive, there is a pocket of low pressure behind the car. This draws air backwards
directly against the rear of the car. This is called the venturi effect.
Secondly, if your rear seals aren't sealing properly, the exhaust fumes get into the cabin.
The issue can be worse if you have a rear facing exhaust like the Brabus exhaust.
This exits the emissions directly into the low pressure area.
You can't change the shape of the car or the venturi effect so all that's left is sorting the seals.
All of the following seals that need to be checked are highlighted in yellow.
The upper glass hatch seals and the mating face against the hatch.
Lower glass hatch seal and the mating face of the hatch.
The engine cover seals...
...and the mating face.
In fact, make sure the engine cover is fitted correctly in the first place.
Visually check the seals for damage and the mating faces for damage or debris.
You can put talcum powder on the seals and close the hatch to ensure proper contact all the way around.
Using seal cleaning solution can help plump up old and tired rubber seals. Click here.
The final solution is to go to smart and buy new seals. The old seals just pull off.
New seals can be bought directly from smart.
If you have a large gap (3mm/4mm) between the upper and lower hatch, make sure that the
seal that runs along the top of the lower hatch is around the right way and not back-to-front.
You do have a certain amount of adjustment of the lower boot flap.
Up and down can be adjusted on the hinge nuts found under the boot carpet.
In and out can be adjusted on either the catches or the striker plate.