It is possible for the light to be on for an almost insignificant reason, the tiny little heater circuit in the first of the two oxy sensor for example, this fault will only delay the operation of the oxy sensor by a few minutes after start up and is really unlikely in reality to affect the car or your wallet.

But, the whole point of the Malfunction Indication Lamp is to indicate a malfunction, therefore, you are likely to suffer from the old "cry wolf" trick, if you just ignore the reason the fault light is on, when another fault comes along, you wont know it... unless it breaksdown of course!

I would be VERY wary of anyone, ESPECIALLY a dealer, who tells you it is ok to ignore it because they cant find the cause, either they dont want to (cause they are underwriting the warranty) or they cant (then they shouldnt charge you for the repair/diagnosis).

I would also be VERY concerned about a slightly raised CO, most modern cars run nowhere near the MOT limits at idle, and if they do for prolonged periods, you can be sure the CATs life will be shortened, CO = partial combustion, which means the CAT is having to finish the job off in the exhaust, which means less bang for the buck (poorer/less good fuel economy) and a lower, albeit slight, power output.

My advice to anyone who puts a any car into a dealer or an independant garage for diagnosis in the event of a SRS, ABS, engine control, etc fault light coming on, would be to make sure they include the recorded fault codes in the final invoice, this will give you something to go on if the fault should return, and a google search will at least give you an insight into other peoples experience of the problem.

At the very least, I would always be happy to research and tell someone what there fault code means.