> So the harm could be CT scans, biopsies, colonoscopies

Colonoscopy is low risk, but not no risk, and there are plenty of people who have a colonoscopy that goes wrong and bamn they're taken for surgery, anaesthetised, have part of their colon removed, and maybe have a (sometimes temporary) stoma fitted. Anaesthesia is not low risk.

Over-testing leads to over-diagnosis which causes over-treatment, and that causes harm.