Replacing the gas cap was the best and most obvious place to start.
First question, when you completely fill the tank, is there any noticeable fuel odor? I hope this is not the case, but 04s are notorious for the crossover tube between the two fuel tanks cracking.
If none, reset the code, and see how long before it returns. (it will likely take at least two drives before it will return). This doesn't fix anything of course, but it helps to know we're working with a current and persistent code.
If/when it returns, remove the left side fuel rail cover, and just under the fuel rail, you will see a hard plastic "squiggly" looking line coming from the rear of the engine. At the middle/rear of the engine, you should see a light green colored cap on that line covering the service port. (red arrow in pic below). Make sure this cap is screwed on tight.
Next, follow that line forward, you'll see it wraps around and enters the intake manifold just behind the throttle body. Make sure that is connected securely. (shown disconnected in pic below)
If all that looks good, and the code persists, leave the rail cover off, go take the car for a ride. Long enough that the car is fully warmed up When you return and shut the car off, immediately pop the hood, and sniff around this plastic line for any obvious fuel odor. This evap line has been known to crack, but may not be obvious by just looking at it.
Failing all that, the dealer will have some specialized tools for this specific issue, and if you do end up having to take it in, it shouldn't be a guessing game at all for them. What they will do is completely seal off the fuel system, and pump smoke in through that service port, where the smoke escapes is where the leak is.
Hope that helps.