There are a few places you can look at in the rear of the car. One place is tail light seals and the other is filler pipe to trunk floor seal. You can also check the condition of your weather stripping for the hatch or trunk.
If you already have the fuel tank removed now is the perfect time to check the condition of the filler pipe to trunk floor seal.
Since I pulled the tank to replace the sending unit and to weld in the rear non existent tail pipe mounts, now was a perfect time to inspect things.