These can be tricky. Looks like you got a bad mechanic. This could be a leaking purge valve, a leaky vent valve, a bad Evap canister, or the fuel tank pressure sensor. Any holes in the Evap lines or rust holes in the fuel neck will also cause the problem. Typically when there is a leak in your fuel Evap system, you may experience the presence of gas fumes on a warm day or when you park inside a garage. Perhaps there is a leak and the mechanic was not thorough enough to find it.

The Evap system is designed to capture fuel vapors from your gas tank and store them in the Evap canister. Occasionally, the computer will command the purge valve to suck those vapors into the engine while it is running to empty the Evap canister of the stored vapors. The vent valve vents the excess pressure in the tank once the vapors have been stripped from the air (removed by the Evap canister). Before the Evap systems were mandatory, gas vapors would vent into the atmosphere. Now it is required to capture these vapors and then consume them in the engine.

Here is a simple diagram of a typical Evap system

Cite: http://www.agcoauto.com/content/news/p2_articleid/294

It is not critical to remedy the issue, but the dash warning light will stay illuminated and you will not be aware of any other issues if they appear. I recommend that you find a better mechanic to properly diagnose your Evap system leak.

Try asking friends and family if they know of a good mechanic. You can also ask the local parts store if they recommend any good mechanics.