Jim,

It's unlikely that a higher octane would cause the check engine lamp to come on...putting the gas cap on incorrectly without a positive seal would however cause an emission fault. Any fault with the ignition or fuel control units will also activate the check engine light. There are some obscure transmssion faults (around a dozen) that can also trigger the fault lamp. I would start by reading all fault codes (if any) at least twice, then clearing the memory. Clearing the memory is done by inserting the DLC probe in A7. Hold down the button for more than 5 seconds. Release button and the LED should remain on after 3 seconds. Press the button again and hold for a least 5 seconds. Release button and LED should go out, the DTCs are now erased and the check engine lamp should be out...

See if the lamps comes on again and/or check for codes in about a week and go from there...