I had that after I moved my evap system to the other side. I undid every hose, added vacuum grease, then added a hose clamp. That was after about 5 smoke tests, and checked every inch of tubing going from the tank, to the evap, to the purge valve and that didn't help. The small leak on the evap system can be really small. I think if the system looses 0.5 psi over 15 minutes it will pop the code (or something crazy like that).