After the brutal fight in Shiganshina wall with the remaining soldiers alive, Eren Yeager, Mikasa Ackermann, Commander Hange Zoé, and Captain Levi Ackerman proceeded to go in basement

When Mikasa accidentally knocks over a cup, she bends down to pick it up and discovers a small keyhole in the side of Grisha's desk. Eren tries his key in it, and opens a secret drawer with a false bottom. Beneath it, they find three books. Eren and Mikasa open the first of them together, and find a portrait of a family fastened on the first page. Eren takes it out

Eren: Is this… a painting?

Hange: let me look closer, no this is far to detailed to have been drawn by a human hand.

Mikasa: Hey there is writing on the back (Hange reading off the paper)

Hange:  this is no Illustration it was made by gathering the light reflected off by the subject and burning it into an image on special paper. Looks quite nice right, where I’m from there are many amazing technologies like this mankind isn’t gone. Beyond the walls countless humans live in comfortable in refinement I pray the person finds this book will be a fellow patriot