I'd call that incorrect. A. If the bird was exposed, there's a good likliehood it will show symptoms in the early stages of infection (which can vary from 3 days-3 weeks depending on the disease). That's the point of quarantine. In most cases, infection due to an asymptomatic carrier is the result of a bird which was infected a long time ago and recovered from initial symptoms B. There are tests for most infectious diseases. If there was still doubt at the end of a long quarantine that the bird may have been infected, it could easily be tested. It costs around $18 to get, for example, a Mycoplasma test around here, which, emotional value aside, is far less than the cost of raising a replacement bird to maturity and so monetarily well worth it. C. Not all birds become carriers. The vast majority that do, show symptoms. The only birds I've seen that came into contact with an infectious disease and didn't show symptoms were the birds so immune that they never even got the disease, much less carried or spread it.