Number 12 knew about magic and agreed with Han Jingru that it was nothing but illusions.
Besides, Han Jingru wouldn't be so lame as to prepare this in advance just to put on a show for him.
But if it's not magic, how did Han Jingru do it?
While Number 12 was still caught up in his thoughts, Han Jingru had already walked into the room and located the source of that stench. It was the birthday cake he had given Qi Bingying several days ago.
Seeing as the cake hadn't been unboxed yet, it meant that Qi Bingying didn't even eat it and merely left it to go bad.