spoilers for A Darker Shore!!
i actually whole heartedly believe that Wylan knew Kaz was using his sheet music for SOMETHING and i would love to tell you why :)
as we know, Kaz used these little dashes underneath the notes to indicate which words to read for the coded message:
and of course, these dashes have an actual meaning in music— they’re tenuti (tenuto is the singular), and they tell the player to the hold the note for it’s entire value (or even a smidge longer). they can tell you to also play the note more legato than the others, meaning you should connect it to the next note.
now there are two reasons i think Wylan caught on to Kaz’s intentions, if not his plan:
1. this note with both a fermata and a tenuto marking:
a fermata tells the player to sustain the note past it’s value. so over a whole note, it means to play the note for at least five beats instead of four (how long exactly is up to the performer). but here’s the thing!! a tenuto and a fermata over the same note makes no sense! the fermata tells you to sustain the note, so a marking telling you to hold it for it’s full value is unnecessary! and even if it was intended to mean the sustained note should be connected to the next, a) a slur would make more sense, seeing as the tenuto’s main meaning doesn’t apply here, b) it doesnt really make sense for just the one note to be connected to the other, when fermatas are typically played with a slight break between them and the next note.
okay now remember that in Jesper’s letter, he says “they debated the subtleties of a note being “sustained” or “held”. a tenuto is held, a fermata is sustained. this particular note and marking was likely the source of Kaz and Wylan’s debate. i’m certain Wylan was arguing that the fermata made the tenuto unnecessary, while Kaz insisted on the tenuto because it was necessary to his code.
2. there are a few places the tenuto marking is placed incorrectly:
with this one, the notes on the bass (bottom) staff have shorter stems than the one on the treble (top) staff. usually, the melody is played with the right hand and shown on the treble staff, but putting the same notes smaller on the bass staff means you can play it with either hand. while technically fine, it makes much more sense to play this melody with the right hand than the left, as the right hand is only holding one other note, while the left it holding two. however, adding the smaller notes where they are means that the tenuto marking can be put there instead of under the treble clef. however, as the notes on the treble staff are the main notes, they should be the ones with markings. putting the tenuto where it is makes it seem like that note should only be played tenuto when played with the left hand, which doesn’t make sense.
with the note stem up like this, a tenuto marking should be placed in the next available space below the note head. here, it’s instead placed above the note stem.
in both these instances, had the tenuto been placed correctly, it would have corresponded to a different line of text, messing up the code. Kaz probably instructed Jesper to place them where they went, and Wylan let him, even though he likely would know these placements are wrong.
Kaz’s sudden interest in music composition would definitely come across as strange to Wylan (in his letter, Jesper says that he himself was shocked). i think that plus Kaz’s insistence on certain things Wylan knows to be unnecessary or wrong had to have tipped him off to there being something else going on. Wylan probably argued with Kaz about the fermata and tenuto for some time before putting it together and relenting.
and let’s be honest, there’s simply no way Wylan would let Kaz make a marking he thought unnecessary or incorrect on his composition unless he sensed there was a reason behind it.