This could be quite the bear to lock down. I know OCD, believe me it is a pain in the A$$ to delve into a minor quirk. Having OCD makes this a 'what-if' nagging burning, annoyance right? All the things it could be magically balloons into something major somewhere down the road (in our minds). So, if you just cannot get over that dang code (burning into your mind) then I would suggest casually (over time) looking into the systems associated with setting that code. Some of the possibilities (even something simple as the loose gas cap or some vent line in the evap system cut/damaged), loose wire, water intrusion, rodent damage. All WILL make it worse down the road (which is why us OCD folks tend to not let it go by the wayside and exhaust options beforehand). Think of it as a long term project, or get over the nagging code issue. Good luck, if you tackle this one, you may definitely need it!