Which criterion might indicate that a test case is needed for further investigation in a software module?

The indication that a test case is needed for further investigation in a software module occurs when condition combinations yield unexpected outputs. This scenario suggests that there may be underlying issues in how the software handles different input values, which can lead to bugs or behavior that is not in line with the expected outcomes. Unexpected outputs often signal that the logic in the software is not functioning correctly under certain conditions or combinations of inputs, warranting further testing to uncover and address these discrepancies.

In contrast, passing all defined tests, running without crashing, and having all requirements validated suggest that the software is functioning as intended. These scenarios typically indicate that the software meets the specified criteria and does not present any identified issues. Therefore, encountering unexpected behavior during tests highlights a critical area for further scrutiny and testing to ensure the software's reliability and correctness.