I have talked about this with my close friend, an ENTJ. We came to the conclusion that just as we believe we're unnoticed by the sensors, because they're not noticing the parts of us we consider most important, only our physical presences and tangible parts... well, in much the same way, we tend not to notice the tangible parts of other people, and we're always busy looking 'into' them, beyond the material and to the inner substance. Because of this, we don't notice and are not impressed by new haircuts, fashionable clothes, nice cars, things like that, which some people want us to notice, so they believe we're not noticing them, because we're often busy looking hard into parts of them that they're perhaps not aware exist, or at least if they are, they don't put much importance in them.
This was a good year or so ago, so I'm not sure how/if my take on that has changed. But it's a definite possibility that N's and S's are, in general, just looking at the 'wrong' parts of each other. We're not noticing the things about each other that the other person wants to be noticed. So we feel ourselves invisible.
I remember my ESFJ sister once telling me that she always got the impression I didn't give a shit and didn't seem to be really listening when she was talking about her latest home improvement or flower arranging or whatever else, but in fact the whole time, though she was right that I wasn't listening to her in the way she expected, I was actually really intently tuned into the undercurrents that she was virtually unaware of, in her own communication. I was able to demonstrate this to her later, when I made an observation about her general way of operating, and gave examples of things she'd told me in the past that betrayed these patterns to me, and she was like 'Wow, it's like you see right into me, how can you know these things? I never told you those things!'
But if she'd asked me what her favourite flower/colour was, or she asked me on the phone to describe her hairstyle, I'd have just made with the Gallic shrugs.