After Granny Wang next door turned into a zombie and joined the Eternal Journey, her granddaughter got into a public renting house that provided better living conditions and was closer to her school. Meng Chao, then, used his contribution to apply for their room and moved out.
They were no longer short of living space, so once Meng Yishan had some money, he decided to remodel the apartment, which led to him digging up quite a lot of the wall plaster and floor tiles.
Coincidentally, Bai Jiacao was at home in the afternoon. She had been bored, so she groped around until she found a box hidden in a corner.
It was a metal box used to store MRE hardtacks around thirty years ago.
At that time, they lacked resources, so when they finished eating the food inside, they were often reluctant to throw away the box, so they used it to store other stuff.