That's interesting! I have a sports exhaust fitted but the light can stay off for a period of time! Then suddenly, it would just come on and stay on until it's reset.
If what you say (about the CAT) is the cause of the light coming on, why doesn't it happen immediately instead of after a period of time [after it has been reset], and after the car has been on several runs?
I think that it could be just an over-sensitive sensor which needs replacing.
