I just realized something...

[u/bersaylis]

I was listening to Ikuta Erika's Bitter Candy Tour and the 2nd verse of "Dakara ne" starts with 「カーテンの」but I recall hearing the same word with the same melody. Turns out it was from "Naimononedari" and it's the first line even but one semitone down.

Dakara ne
Naimononedari