[Davie Boy]

I've owned two '88 924s over the years. For both cars, I had to replace the tube running between the fuel tank and filler cap. In both cases, I knew there was an issue because there was a fuel leak visible from the exterior of the car. Maybe you have a slight leak here. The exterior vapors may be evaporating quickly but - if your hatch isn't sealed very well - the interior vapors are getting trapped due to the lack of ventilation. Just a guess.