That's what makes the statement "no 'unknown unknowns'" a bit ridiculous though. How can you possibly assert that there are absolutely NO 'unknown unknowns'? You can never rule them out, and at best can only suspect there's a possibility either way, that they do or do not exist. You can never actually know with certainty.
You can have a statistic approach, and that's what the OP is referring to. If you find out about an unknown unknown frequently, they are out there. If you go years without hitting one, they are (probably) absent.
Schrödinger’s Cat?
I suppose the programming equivalent is something that randomly results in a line being printed to the console.