[dragonwriter]

> Absence of evidence is not evidence of absence

But it is also not evidence of the thing for which there is absence of evidence.

EDIT:

> especially when searching for evidence left behind by competent adversaries (e.g. NSA, GCHQ, etc) who have a strong motivation to remain undetected.

No, there is no “especially”; absence of evidence means no basis for any affirmative belief, period, equally for any fact proposition. Arguing for “especially... ” is exactly arguing for a case where absence of evidence is evidence for the thing for which there is an absence of evidence.

[anonym29]

I'm not asserting that it is.

In risk management, you shouldn't ignore known unknowns like that, you should either adapt your threat model or risk accept, not simply consider that risk nonexistent until proven.