just a little update. We got the car back from the dealer today and got the problem fixed. It ended up being a pressure sensor in the Evap Canister. (it is part of the fuel system.) There are apparently several causes of the "check gas cap" message. Either:
1. a real leak in the fuel system causing a loss of pressure in the fuel tank.
2. A bad pressure sensor (there are five of them according to the dealer. IF any one of them goes bad, the "check gas cap" message will be displayed.
3. The gas cap is loose causing a pressure loss. Because the gas cap is something that anyone can check, that is why the DIC displays the gas cap message. When the car realizes the pressure is not re-gained, the Service Engine Soon light is illuminated so that you take the car in to the dealer to have it fixed.. this problem will NEVER leave you stranded. It'* an inconvenience more than anything.