What vehicle is this? Some newer ones have been having this problem. If the cap is not on tightly and allowing vapors/air to be released the system notices this pressure change and trips the light. This is mainly emission related.
When the tech reset the CEL did he/she pull any codes???
I'd recheck the cap seal, ensure that it is servicable and on tightly. If bad, replace.
If it is good and you are sure that it has a good seal, check the emission lines connections for leaks and holes. And the emission related components.