First off, you don't need to pay anyone to check your SES code. You can take your car to most auto parts stores (i.e., Advance, AutoZone, Pep Boys, etc...) and they'll do the diagnostic for free!
I would get the code diagnosed just to confirm that it's the same issue, but considering the circumstances you just described it sounds very much like it could be a faulty gas cap. Just curious, after how many miles of driving did the SES light come on again for the second time? If it was less then say 25 miles or so, then I would presume that it really wasn't fixed properly the first time around. A lot of times, certain trivial SES codes (an imperfectly sealed gas cap being one of them) can be manually cleared, and without even fixing the problem the SES light can stay off for a certain number of driving miles, hence giving them impression that maybe it was an erroneous trouble code or perhaps the mechanic fixed it but really didn't do it right to begin with.