2001 VW Passat Gas Leak

I have a 2001 VW Passat with 43,000 miles, automatic trans, 2 wheel drive. I have a gas leak in front of the rear right side tire dripping from the guard shield. This only happens after the car has set idle for a day or two and does not begin to drip until the car is started and the car begins moving. Each time about a 1/2 cup of gas leaks from the guard shield I removed the guard shield so I can see that the gas leak is coming from somewhere around the area of the canister. I then disconnected the canister and about 1/4 cup poured out. I only have 1/4 tank of gas and I haven't filled up in over two weeks so I don't believe it is coming from over filling the gas tank. My question is, "Where is all this gas coming from that keeps filling the gas canister?"