If one thinks outside the box , something similar is one possibility.
Police trying to solve the puzzle would have to consider at least momentarily a similar scenario.
Otoh. They have the notebook and access to his computer which will either enforce or rule out the possibility.
Oh yeah, procedures and evidence will usually get police to the correct conclusion. As you pointed out that notebook may also be a treasure trove of information.
If he was a bad actor I hope his little notebook (journal?) names, names and dates and everything else on a large number of human scum.