Medical word for finding an unexpected/unrelated condition

"an incidental finding"

• Incidental findings are previously undiagnosed medical or psychiatric conditions that are discovered unintentionally and are unrelated to the current medical or psychiatric condition which is being treated or for which tests are being performed.

My GP (general practitioner; family doctor) mentioned the following today:

Incidentaloma

A mass or lesion noted during examination performed for other reasons.

It is common medical slang in imaging and radiology for when the patient complains of, say, an abdominal problem but during imaging a little spot is noticed outside of the abdomen.