1998 Mercury Sable gasoline odor

1998 Mercury Sable, 99,000 miles. I just purchased this car and it seems to run fine. However, on several occasions I have noticed a distinct unburned gasoline odor in passenger cabin. In a parking garage this week, I noted the smell of unburned gas while standing outside the car in front of the engine compartment. There aren't any leaks on my garage floor and car idles and runs smoothly.