I think it was the dorkiness that he displayed at times and maybe just coming across as a bit happier, even if it was shown that it was partially an act.
I suppose not all of these need to mean it's him, but this is what I recall of the character's appearances thus far.
-Turned on the intercom which got everyone to hear that Haise was getting tortured by Taki and led to him getting rescued. Was shown like it was just some random accident (and it certainly could have been just that), but in the first series Hide demonstrated using seeming accidents to do other things like how he got that bug on Yakumo.
-Was seen rowing randomly from Rue Island. No real evidence of what this could have been, though given we're later shown that Amon knows him it might have been him dropping Amon off on the island.
-He was the one who helped get Amon out and it came across like Amon knew him. The way he took out the guards also wasn't necessarily the way a ghoul might have done it, but of course that doesn't necessarily mean it couldn't have been.
Hide demonstrated impersonating a ghoul before and him pulling this sort of thing off wouldn't be out of character. While Kaneki believes he killed and ate Hide at Hide's own request, we've never really been given complete confirmation he died. Kaneki just assumes that to be the case. I don't have doubts Kaneki took a bite out of him, but him being alive isn't out of the question. There's also Marude implying Hide was the one who put the idea in his head that the Washuu's were ghouls by checking the scanner, though we don't know when that conversation took place.
I'd actually forgotten about this one, but Haise got that package that had the mask as well as that signed copy of one of Eto's books that Hide had gotten for him way back when. Uta could have just sent that too, but I'd wonder when he got his hands on that unless I'm forgetting a scene.
Most of it is probably just looking at it a certain way to make it fit the theory, but the character is rather mysterious and offhand Hide seems the best candidate I can think of. It would be funny if it was someone completely different and Hide's just straight up dead though.
