It sounds like it’s a EVAP code, but post it/them here. Your last bit of information strongly suggests this.
I don;t have access to the documents for your particular car, but I’ll post my own just for illustration of my points.
The gas tank is designed to be able to breathe in and out through a charcoal canister. The canister catches hydrocarbon molecules. The emissions system has a pressure sensor that measures vacuum in the gas tank. A slight vacuum is maintained to prevent hydrocarbon molecules from escaping. If that vacuum gets too high or too low, the pressure sensor tells the ECU and a Check Engine Light illuminates. The fault codes stored in the ECU then tell the tech that the EVAP system has a problem and gives him an idea what it might be. The idea is that he’ll diagnose from there. The problem is that far too many techs don’t understand the EVAP systems well and don;t want to work on them. So they attribute every leak to a “bad gas cap”.
The tech should be able to access for your car an on-vehicle inspection protocol for the system, similar to the one I’ve attached. Don’t be afraid to ask him.
In the meantime, if you’ve been “topping off” your tank, it would be a good idea to discontinue this practice. In some older vehicles, fuel can get into the charcoal canister and prevent the system from breathing on as the gas is pumped out.
http://tijil.org/Scion_Docs/Scion_06_misc_docs/2007sciontc_ncf%20Folder/2azfeemi.pdf
http://tijil.org/Scion_Docs/05_tC_Shop_Manuals/Repair%20Info/Repair%20Manual/Emission%20Control/Emission%20Control%20System/onvein40.pdf