Two possible "suggestions"
1.. Introductiion via secondary source: Corals, rock, LFS
water or invert shells which I would think one of these moreso to be the cause.
2.. The fish have been healthy enough to fight off a complete infestation (not very likely though IME
). Ich cannot really survive without a fish host past 12-14 hrs as a theront
. If the fish have been QT'd when first purchased, ich will most certainly surface and can be effectively treated. If treatment is needed and properly administered, it is impossible for that same fish to be the cause of any future infections.