Complex sentence: In 90% of cases an undescended testis can be felt in the inguinal canal; in a minority the testis or testes are in the abdomen or nonexistent (truly "hidden").
Split into simpler sentences: In 90% of cases an undescended testis can be felt in the inguinal canal. In a small minority of cases missing testes may be found in the abdomen or appear to be nonexistent (truly "hidden").