engine light..small evap leak

Evap code says small leak in evap system.

I bring to my local mechanic. He first advises me to try replacing the gas cap. No dice.

Then he performs the gas tank smoke test and doesn't see a leak. He basically says he doesn't know what the problem is.

So I take it to the Pontiac dealer ( always a last resort). They say my vacuum harness ( part number: 24507476) is bad or leaking. So I go to gmpartsdirect.com and order it. My mechanic throws it in and resets the code. No problem for over a day....then the engine light comes back on. I'm at my wits end with this problem. Any GM trained mechanic out there see this before ?

Happy to report this problem is finally fixed. Summary on the invoice said, "Vent systems performance. System not relieving fuel tank vacuum. Remove and test vent valve, purge valve stuck, replace purge valve, test drive."