What is source of gasoline smell outside car, only with engine off 2-3 hrs.? on 2004 Saab 9-5

the smell is intermittent and sometimes after non-use overnight;
no codes on it and nothing showed up on system pressure test;
sometimes no smell for weeks then starts again; gas cap ok; worse in hot weather; full or low gas tank no difference

bad exhaust/catalytic converter?

Asked by for the 2004 Saab 9-5