<p>To be honest, shouldn't Hinata see through Obito's mask and see his face?
</p><p>I guess we can conclude that Obito's mask is special. Like why not? It's not exactly like he wears different types of mask, in a world with Byakugan users, chakra sensors and what not, I feel Obito would choose or make a mask with special sealings to it, Obito's smart and resourceful enough to do that.
</p><p>And about Chakra, maybe his chakra changed? I mean Uchiha chakra gets darker and becomes unrecognizeble, like how Karin stated about MS Sasuke,(plus we need to include Karin in this list, since she should atleast seen the eye chakra of Obito and Kakashi), Shisui's case is that he never changed, personality wise, meaning his chakra stayed the same, which Ao could recognize from the war itself.
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