> 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.