And I will give credence to the idea that they cannot be so obvious or else the killer will simply erase them, even though I am of the firm opinion that it would be ridiculous for a killer to leave any message behind, particularly if it is too cryptic to understand.

Hmm. Alternatively, there is the option of a cipher. The one I typically use would be too complex to teach someone else to decode in such a short time, but it could be modified. Though - it makes me wonder if that's what Grace meant all along. Luca was a mathematician, was he not?