Fuel system leak detected???
First things first.....The whole fuel cap very seldom needs to be replaced in it's entirerty.. There is a O-Ring on the under side that can and does dry out and break or become dislodged and won't seal any longer.
In view of the fact that your car has already been scanned and the original complaint was not diagnosed or repaired correctly, should mean another scan at N/C under the dealers service warranty which is usually 12 months/12,000 miles. The service techs can attach either a vacuum or low pressure source to the pressure test port (IM 240) to help find the leak. The dealers service tech. should have done the obviouse repair, cleared the codes and road tested your car for at least 10 miles to make sure that the condition was actually repaired correctly. The scanner is the first line of repair for diagnosis, and the second line of repair for re-checking for repeat codes thrown after the fact just to confirm it was repaired correctly.
Take your car back to the dealer with your invoice and complain that the CEL light is back on and that it came back on right after you left and that you need them to "really" fix the problem this time. Like I said, it should all be covered under the original repair warranty...Good Luck, JK