Look for contradiction, not outliers
Outlier hunting produces volume and little conviction. Contradiction produces few candidates and high conviction, because a combination that cannot exist does not need a threshold to justify it.
A payment released against an ended relationship, a credit issued against a charge never raised, a service billed outside its authorization: none of these require a judgement about what is unusual.
PERISCOPE briefings describe investigative principles. They do not disclose detection logic, thresholds, formulas, weighting or rules.