could be flukes they can draw their body up under the scales so a person may not think to much of it. ICK dose not disappear when the light is on shrimp and wrasses may pick it off. dip in fresh water in any case. and QT with prazi pro if you can see anything floating on the tub you used for dip. The fresh water dip has to be for more than 3 minutes other wise it is a wast of your time. If the fish seems to be really stressed in the dip then put him in the QT. Freshwater dip tub should be at the same temp as the tank and the Ph has to be the same. Put a air stone in the fresh water before you start dip remove it before you place the fish in and if the dip is going to be longer than 3-5 Min's put a air stone in with the fish after he has adjusted to the dip this is less stressful. IMO.

The only other parasite I have seem do this disappearing act is Brook. You would need to rule out fluke anyways so fresh water dip. treatment for brook is formalin and you have to do this now if you think it is brook as it can kill as little as 24 hrs to 48 hrs from first visual signs. I always feed my fish with selcon and garlic they love it and it helps them.

Bill