While playing as Satoshi and Naomi in the second wing, they will come across Sakutaro's cell phone. It contains many images of the corpses he has come across in his time searching for Mayu, as well as a picture of Mayu's corpse. Somehow, the cell phone was able to record his final moments of death.
EXCHAP 2
Sakutaro will use his cell phone in the second extra chapter to take pictures of Mitsuki Yamamoto's and Kaori Kimura's corpses. While walking he will encounter Yuuya and drop his phone. Yuuya picks it up and manages to see what Sakutaro was doing with it.
Significant plot details end here.
Trivia
There is a graphic of Morishige's cell phone without blood, that can be found in the game's data. However this item's graphic was never used in the game.
Sakutaro keeps all the pictures of corpses in a folder called either "corpses" or "corpse"
Picture 33 is a picture of Mayu that was taken outside of Heavenly Host. This is also the last picture he takes in the real world, meaning that he took 134 pictures in Heavenly Host, 65th, 67th and 68th seen as being pictures of corpses and 62 being a picture of Mayu's corpse.
Interestingly, 66th picture doesn't exist on his phone, as it is skipped completely when he is going through the pictures.