A small fuel leak can do this. I had this happen on a BMW and it ended up being the fuel filter connection dripping onto the lower control arm, where the fuel was evaporating. Difficult to diagnose (took me about 6 weeks to finally locate it).
If it is raw fuel that you smell, if it it under the hood, check all the fuel fittings. Even if you don't see a leak, you may see a dirty fitting where the fuel is causing dirt to stick. If the smell is stronger in the back of the car, it could also be an Evap system issue.