Like Joe said earlier, Ich cannot go dormant.
It either dies or it feed on a part of the fish that you can't see (Gills)
the white spots you see are not the ich itself, its a sort of scar where the ich has went in and burried itself inside the fish, what you are seeing is basically scar tissue.
The likley reason the ich never came back is due to your fish being helathy enough to fight it off from being able to penetrate the exterior skin of the fish (slimecoat) so instead (if it is still present in the system) It attacks the area of the fish that is always vulnerable, and that is the inside of the gills.
It could be in there reproducing for a long time and you would never see it.
Then you have a power outage, or add a new fish that causes other fish to stress out, or a heater malfunction and your fish get stressed and thier slimecoat is weakened. Then all the ich that has built up over time attacks.