[Update: Word has gotten out that the Persona 3 dorm assets were not a discovery, but actually someone's custom mod work. I was fooled like many others by the internet. Sorry for getting anyone's hopes up, but this is still pretty cool.]
Yesterday, a YouTube user posted a video showing the lobby floor from the Persona 3 dormitory buried off in a corner of Persona 5.
No textures are applied, but it's certainly the old dorms. All that's missing is Akihiko talking about protein.