In Jujutsu Kaisen's episode 16 "Kyoto Sister School Exchange Event—Group Battle 2—" (Transcription: Kyōto Shimai-kō Kōryū-kai—Dantai-sen ②—; Japanese: 京都姉 妹校交流会—団体戦②—), during a flashback scene explaining Panda's origin, we have the following conversation between Yaga Masamichi, principal of the Tokyo school and Panda's creator, and Panda as a toddler or "preschooler":
Yaga: Panda. Actually, you have an older brother and sister.
Panda: I don't.
Yaga: No, you do. They are in your body. <mutters to self> What's with the honorific?
What exactly is the honorific that Panda uses here? I am somewhat aware that there is a hierarchy of formal speech in the Japanese language, but I'm not able to precisely identify what honorific is being used in the Japanese dub. Is it perhaps the —masen ending used by Panda? Furthermore, what is unusual about Panda using that kind of speech with Yaga, prompting him to comment upon it?