Without the exact DTC codes, it's pretty pointless to guess WHY the CEL came on and what diagnostic procedure was followed.
A loose or leaking fuel cap can cause several DTCs, namely a P0442, P0455, or a P0456. All of these have mulitple causes, the cap is only one possibility.
Since my tech library only goes to 01, I do have a question. Do the 02's and up have a separate GAS CAP light in the dash? If so, can someone who has the later service docs look up the circuit and describe how it works? I'm curious.