[projectRDM]

The ECU has a two trip fault cycle. It will pick up a code if a condition is apparent but not trip the CEL if it goes away within a certain time limit, the code is then stored for future indication if the condition is seen again. After 30 cycles it erases it completely.
So yes and no. There may be codes present in the ECU, but they're not a concurring problem if the CEL hasn't already been tripped to indicate them.
Also you may want to point out to him that a lot of engine problems won't throw a code at all since they may not relate to emissions output, so it's possible that there never was a code present.