1999 7.4 PO442A Code

I recently got a PO442A code reading from my first check engine light with 89k miles. (1999 2500 7.4) I had the fleet mechanic at my work look up the code, which he says is "Evaporative Control, Small Leak Detected". Of course there are a million possibilities as the cause. Question is, is there a common cause that I can look into ? anyone else have this code ?
I should also note that I never fill my gas tank while the truck is running, and the cap is always on tight. :banghead: