Hmmm. So in that situation, the Assassin has the belief "The Scum is an unsanctioned psyker", justified by the observations that (a) the Scum is not a sanctioned psyker, and (b) the Scum just used a psi ability. But in a world where mind-swapping is actually possible (and moreover can happen unexpectedly and without obvious external signs), we have to expand that reasoning a bit and expose the implied assumption, so that it goes more like: (a) the Scum's mind is not a sanctioned psyker, (b) the Scum's body just used a psi ability, (c) we presume that the Scum's body is currently occupied by the Scum's mind. Of course the error in the reasoning is (c), and for all that it's an understandable assumption if mindswaps are possible but very rare, it's still inaccurate.

(Also, the above reasoning only yields the belief that the Scum's mind is an unsanctioned psyker; that must be combined with point (c) a second time to reach the conclusion that it's legitimate to gun down the Scum's body right now. That latter conclusion is certainly not knowledge, since it's not even true. Only the intermediate conclusion about the Scum's mind constitutes the sort of quasi-knowledge we're discussing here.)

I think I'd agree that it's not clear that this fits into my suggested unifying pattern.