So observing a lack of sabotage increases the probability that no Fifth Column exists.
No. Observing nothing carries no information. You can’t update the probability of the existence of a Fifth Column (that doesn’t sabotage or hasn’t sabotaged yet) based on the lack of sabotage. Sure, you can observe nothing and update belief and reduce the likelihood of possibly of the existence of a Fifth Column that would have (with some probability) sabotaged by now.
It’s a mistake to say that lack of evidence is evidence of anything (the governor’s position).
It’s a mistake to say that lack of evidence is evidence of anything (your position).
There was, as far as I know, no observed evidence of a causal link between smoking and lung cancer.
The correlation (specifically, the relative risk ratio) was roughly 19,000% (~190x).
There have been some “natural experiments” that suggest a causal link since, but I don’t think there were any available at that time.