Last year I bought a 2000 Outback Limited and it has the same problem. My mechanic tightened the screws on the fuel line clamps, but he told me this is a common problem and he knew of no permanent fix. What I’ve resorted to (and what he suggested, actually) is carrying a phillips head screwdriver in the glovebox during the winter months, and whenever I smell the gas vapors I pull over, pop the hood, and inspect the fuel line clamps for moisture. More often than not I find the culprit and tighten it. I tend to tighten all of them while I’m at it.
Good luck!