2000 Infiniti Q45 gasoline fumes inside car

For several months I have been smelling gas fumes inside my car. This happens at least 2- 3 times per week when I start to drive and will last for about 5 miles or so. On 4-17-10, I had valet parking and when the car was returned, the odor was stronger than ever before inside. When I opened the windows it got worst. Again, after driving several miles, the odor went away. It was 2 days later when I smelled it again. Someone has checked the car, but cannot find anything.