Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
Can you give me the "p-code" that was retrieved?
Ok- when the check engine light comes on, it means there a problem in one of the 100's of parts and/or sensors that are monitored by the PCM. To take a "guess" at which one isn't very efficient. If the mechanic you went to turned the light off with a scan-tool, there had to have been a "p-code". With the use of this p-code is where the diagnostic procedure begins. If you can get that p-code, I can more accurately and efficiently diagnose the problem.
For now, to answer your question- as long as you are not experiencing any drivability issues such as stalling, skipping, shifting problems, etc... then more than likely the problem is an EVAP (emissions) related problem, in which case, it is perfectly safe to drive the vehicle. I DO suggest you at least have the p-codes read, just to confirm this. You can have p-codes read for FREE at any local AutoZone location.