I like some of the other Theories here too, but i thought i would add a few more,
Since Tobi's eye and Kakashi's eye are the same, and if we assume Tobi is Obito then maybe Tobi can see through Kakashi's Sharingan as well.
Kakashi is always hiding his Sharingan ( unless in battle). Has the reason for this been fully explained yet? I don't remember if it has or not.
And if that is not the case, maybe Tobi has mastered his eye to a level in which he can see through Kakashi's eyes
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