Negative Cue
The absence of something (some detail, signal, or anomaly) which highlights an answer to a problem or question.
Doctors used to believe all disease manifested as something foreign/outside the body attacking it. In the case of Scurvy, a lot of doctors got tunnel vision looking for a positive cue. (Some Germ or pathogen present that would explain the condition.) But none was to be found. It wasn't until biochemist Fredrick Gowland Hopkins considered what was absent (a lack of vitamin C) that the true cause of scurvy was discovered. This absence was a negative cue.