most ECU's only turn on the light (set a DTC) when a sensor is 'bad' where bad means out of range....the delphi example is an ECT that is ~ 50 degrees off but still in a "legitimate" range. so you need to verify the sensors' outputs with the scan tool....
The other thing is that the ECU doesn't monitor everything, and the list of what it does check varies with the ECU/model year - newer ones do more than older ones....so a problem may occur w/o and DTC....
I'd be inclined to start by removing the IAC unit and cleaning it.....10 years of time would be a decent service interval....