>If finding something that otherwise carries no symptoms is best left untreated, then the fact that you found it should make no difference to the decision. The doctor should say that the best course of action is to do nothing. More information can never be harmful.
Absolutely and demonstratively false. There is an entire field of health policy that destroys this harmful idea. First of all, just because something doesn't carry symptoms NOW doesn't mean it won't carry symptoms LATER, but doctors can't always predict this and aren't perfect decision making machines. Many times the patient will push them for further tests and treatment (or the doctors will advocate for it to ease the mind of the patient) which leads to potential harm from unnecessary treatments. And this is just ONE of the ways patients are harmed by unnecessary screening. There is also the psychological damage of having a condition you wouldn't have otherwise known about, and living with that knowledge (take aneurysms for example). Psychological stress has a real physical toll on the body. Then there's the COST associated with unnecessary screening and treatments, which (especially in the US) can run into the thousands quite easily for even simple interventions as a result of unnecessary screening.
Even after that, a screening process can also find something that could be bad, but in some patients doesn't actually decrease their lifespan or quality of life. If we have no way of adjudicating between these cases or predicting which cases will end up bad if left untreated at the present moment, what do you think will happen? People will ask to be treated anyway, and iatrogenics will rear its ugly head.
You cannot ASSUME that screening is automatically good. It must PROVE itself as such in a randomized clinical trial. This trial must show that people live longer and/or better as a result of this intervention. In many well documented cases, this turns out NOT to be the case, which entirely destroys your original rebuttal.