As the commentary mentions, this is a slightly modified version of one of the scenes from Karin's love song vignette. I actually did full translations for all of the vignettes (available here), so I basically lifted the dialogue straight from those.
*Smirk~*That one's a secret from everyone else!!Understood?*Glomp*Oh nooo~ ♡Hehe! Yeah, I know~Randomly passing by Yuu-chan.What else~? Oh, what about... the sleepyhead Karin-chan?