Use when you're about to act on a "there is no X / none exists / not found / nothing handles this" claim — especially one from a delegated subagent, research handoff, or a teammate's summary. Positive file:line claims are citable and checkable; absence claims are only as good as the search pattern that produced them.…
Use before claiming something works, is done, is safe, or is fast — verify through the actual trigger, real data, and representative conditions, not a convenient stand-in. Triggers on "verify", "confirm it works", "is it done", "benchmark", "ETA for a long run", "is this edge case reachable", "test the hook", "smoke…