This is indicating a leak in the "Fuel Evaporative" system, which includes the fuel tank, charcoal canister and a bunch of hoses and a couple sensors. The fuel cap is also part of this system so you want to make sure that is in good condition.
What you should do is find a shop that can perform a smoke test to determine where the leak is occurring. These systems can leak from many places and it can be very difficult to find, so the smoke makes this much easier.
Call around to find a shop that has the smoke leak detection machine and have them look at it. This should be a about 1 hour of labor to look into this, more if they need to drop the fuel tank or remove other items.