Absence of evidence is [pretty much the weakest possible] evidence of absence. It is most useful for things which are readily apparent. It is not particularly u

Absence of evidence is [pretty much the weakest possible] evidence of absence. It is most useful for things which are readily apparent. It is not particularly useful for determining questions where the evidence is not readily apparent (such as, obviously, 'Is the NSA spying on Americans?'). One may easily use the absence of evidence of woolly mammoths to conclude that woolly mammoths are extinct. Given their physical size (they are easy to detect) they could not remain hidden from view. We can assume (from our knowledge of other large mammals) that they would leave signs of their presence wherever they trod (footprints, dung, etc.) It is not so easy to use absence of evidence to disprove the existence of neutrinos or tree shrews. Both neutrinos and shrews are hard to detect. There are few people even qualified to attempt to detect them, fewer who are interested, and even fewer who have the resources and opportunity. A relevant question is "Is it possible for spies to conduct espionage undetected for an extended period of time?" The answer is yes, there is ample evidence of that. Schneier is clearly in a place of trust in his community, making him a high value target. The standards of evidence for betrayal of trust being high (and difficult to define), mean that Schneier could potentially be a spy/collaborator in some capacity, and/or could become one at any time.