The Piano.. ( Spoilers )
I've only finished the game fully once, I'm on my second play currently. The most notable thing so far to me was Monika. She tells you she plays piano yet looking through the game files will show you no piano sprite exists. I think when you hear piano music in the back of the audio it's monika watching you (Since she's self aware and can control the script) When she's on screen the piano stops because she no longer needs to watch if she's there. This would also explain why it gets distorted at times. Anyway it's just a theory.