Thinking about Peter knowing how to play the piano, but he doesn’t actually know any premade songs because there’s too much potential connection in that, nor could he play anyone else’s sheet music because he’s completely forgotten how to read sheet music that isn’t his own. Over the years, he’s developed his own musical notation system that only he can decipher. Anything he plays comes completely from his own head and is never shared with anyone. 

Anyway, Elias hears Peter playing one time and enjoys it, but ofc Peter doesn’t want to perform for him (that’s ~connection~ Elias, ew), so Elias has to be sneaky about listening.