1. Didn't find a system description in the F/A-18 -1, but on all the airplane I know which are equipped with ice detectors, they are connected to the de-icing equipment.
If you turn it on, the warning extinguishes and if you turn it off while the icing conditions still exists, the caution will come on again.
2.Another icing myth is that it cannot develop in clear air. While unusual, this is not entirely true.
One area where you may witness clear-air icing is in the vicinity of convective storms...http://www.etcadvancedpilottraining.com/its-just-a-little-ice/