If I doctor performed a cancer screening on me, for free and without me asking, then yes — as a matter of courtesy I would still expect that doctor to tell me if he found cancer, rather than reading about it on his blog later.
> If I doctor performed a cancer screening on me, for free and without me asking
But that would never happen, so the point is moot.
That is legally allowed as part of studies, to which a patient must agree after being proposed. Otherwise, it is illegal.