I think I know what you're talking about, sort of.
I remember once when I was young-ish, somewhere around 10-12, I was in a mall (not of my own choosing), sitting somewhere waiting for some people I was with to do something so we could leave. I was watching all the people going by, when I suddenly was struck by a realization that I had never thought of before: That all these people, and everyone you see in the streets out in the world, are in fact other people. They have lives, thoughts, wants, fears, all the shit that you live with.
Maybe this is something that everyone takes awhile to realize, but I had never thought of this until a relatively long time, and it certainly didn't stick. I'm aware of the fact that, as far as I know, other people are other minds, but it isn't something that registers with me. I don't really into "people-watching". Other people seem like noise, like parts of the environment on the peripheral of my experience of the world.
When I do pick people out of the scenery though and analyze them, I really do wonder. A couple weeks ago I was out at night downtown in my city on a Friday, so that was when everyone was out clubbing. I've never been one for that scene (I wouldn't be posting on this board otherwise right?), and it was really strange to me seeing all these people clamoring to get into a dark building with loud, derivative music, overpriced drinks, and shallow people.
Not to sound like an /r9k/-tier faggot complaining about normies, but if I think hard enough about other people I do wonder how real they really are. How much is individual identity, how much is regurgitation of ideology, or whether such a distinction is false to begin with. Ultimately, it's hard to argue that other people are anything more than extremely intricate robots made of flesh and bone, convincing but ultimately hollow.