Sure, what you're saying is that it goes undetected, that's just like the camallanus scenario. But it must have first rode in as a parasite. Setting up a new tank with no real livestock will soon be loaded with bacteria that are capable of causing infections once fish are introduced, then become stressed and immunity drops. But there will be no parasites from the get-go unless it's a freak case, like if you cross contaminated it from an infected tank with a net, then added livestock which it was able to latch onto before dying.
If someone sees ich out of nowhere, then it could have rode in on a fish they bought long ago, then your theory of it existing unnoticed (which makes total sense to me, apparently some fish have a better resistance) would explain why it seems to come straight from the tank. Because in their mind there are no new fish, therefor it must have come from the tank.