because its not an ending. its a side scene, I think you have to sleep with Kana atleast once and have slept once in your room and seen Ai masturbating in the opening days along with collect objects. It was probably some S&M gear or toy.
It's not a major scene so you can unlock the gallery via someone's save or something if you want to view it.
edit:
Nobile1701 here's a hint, talk to either of them and interact with them as the days go to trigger it