For formal, technical purposes (e.g. when discussing musical audiation and other aspects of musical cognition) the terms "acoustic event" or "notated event" or "vertical event" is pretty much standard terminology within psychology of music for referring broadly to any individual single tone or simultaneosly experienced combination of tones (i.e. an individual chord) within a passage or composition. An acoustic event can also include a silence. Really useful all-inclusive terms, (though not yet in  musicians' common parlance)!

So, I'd  use the term "notated event"  (or the more generalized "vertical event") to fill in the blank in the OP's question.

Is there a general term for a single note or a chord?
In the TV show "Cold Case", the investigation in the episode "The Boy in the Box" (2004) was initiated by someone leaving the box of memories of the murdered boy on the steps of St. Emilian's Church. Among them was a picture of the never-before-identified victim, with the note "His name was Arnold".

My question is: who sent that box, and why then?

My best guess is that it was somehow related to that boy's mother's death (who happened to have "died recently", as sister Vivian said), but I fail to see how. Is there some hint to the answer that I've missed? They usually explain that kind of stuff.
