Work nobody outside your business ever sees
Your customers have no idea this work happens. It exists because two systems won't talk to each other, or because the only copy of the procedure is in somebody's head. Hand it to software and the cost goes away. Nobody outside your business notices a thing.
How to spot it: close for a week and it would be sitting there waiting when you get back.
Its check: the wrong-answer test. Is the job the same shape every time? Could somebody tell you how they would spot a bad result? Could a bad result reach a customer before anyone noticed? Fail any one of those and the job is out, however much it's costing you.
Re-keying orders between systems. Rebuilding the same report. Chasing suppliers. Compliance filing prep.