From my understanding, ich is always present. It only becomes a problem when fish are under stress, weakening their immune system and opening them up to infection.
Unless ICH is immortal it cant survive once the tank is treated, I had ICH in my tanks, my stupid fault for sharing tank maint tools. I treated the tank's with heat and that killed the ICH end of story, With no mature ICH to breed how can it come back unless you reintroduce it somehow.
When we had the big floods here in March the temp in my big tank dropped to 16 degC and stayed there for 5 days while the fish were stressed no ICH.