Q&A

When you pull up the back seat, there are two round black covers, did you pull those covers off and inspect the fuel tank? Those cover seals can leak and sometimes you will see signs of them leaking, sometimes you won't.
My suggestion is to find a good repair shop that has a smoke machine for detecting evaporative system leaks. This should pinpoint the leak fairly quickly.

I know this may be a little late; not sure if you already got this taken care of but I figure it was worth posting: Hopefully by now you've received notice from Volvo of their Recall for fuel pump issues. I'm pretty sure that the 2002 yr S60 was included in the affected models. I have a 2001 and have had the same problem for just about the same amount of time. Your best bet is to take it to your nearest Dealer Service Dept. They may feed you some BS about how they're doing the Recall "in waves" and you have to wait until they get the parts in, but the truth is that the Recall's potential problem has already progressed into the ACTUAL problem in your case (assuming it's the fuel pump).
If you need more info on the Recall or want to check to see if your model is included, have your VIN ready and call (800)433-8263 [8:30am-5pm EST].
In the meantime, a temporary solution I found that helps to mitigate the fuel smell [somewhat] is to only keep your tank about 1/3 full. Sure, it's annoying as heck to make extra trips to the gas station, but it beats driving around and getting nauseous & light-headed from those fumes.