After turning on car with defroster and fan on and having it warm up for about 15 min. there is always a smell of gasoline inside the car. We cannot seem to detect the cause.
If you want a useful answer, you need to give us some information that is currently a mystery:
Make
Model
Model year
Odometer mileage
Engine size/type
Whether it has been maintained according to the mfr’s maintenance schedule
Typical ambient temperature on the days when the odor has been detected
My guess early 2000’s Subaru.
That is very possible, but we should not have to guess.