:| that sounds like a bit of a stretch, but whatever. I think the most probable thing is that Kaneki just ate him
Sidenote: Has anyone noticed that as time goes on more and more of Haise's hair is turning white :|
In case you're still interested, here is my theory:
" have you ever thought that back then when Shirazu saw Touka visiting someone at the hospital, that was most likely Hide?
The last time we saw him, it was implied that Ken had eaten him but that wasn't certain, Ken could have simply taken a bite or something.
In my opinion, what truly happened was Hide making an agreement with Arima that he would deliver him Ken only if they wouldn't kill him, as a last resort to keep him alive from that chaotic situation that took place in the battle.
However, Ken probably hurt Hide and now I think that he's in a coma, explains him being in a hospital 3 years after the incident, let alone the fact that since Shirazu saw there Touka, when they were visiting Shirohara, a man also in coma, they could have been in the same ward exactly because of being in the same clinical state.
The reason for which Touka is visiting him is probably because she feels guilty since she knows that Hide truly cared for Ken and also because for her, Hide is the last bond, her last connection to Ken of the past."