I don't really think - unless everybody else is also screaming in that room - someone standing in the middle of the room and screaming is invisible. Usually, people have noticed that screamer, but have decided to ignore him for one reason or another. If you think in terms of "screaming" being any number of the things people do in order to stand out and be noticed, the act of doing that kind of thing just shows the person is yet one more of the "zillions" of other people doing something in the hopes of getting noticed.
In a world full of people who want/need attention, the person who is just himself and who isn't trying to get attention is often the different one in a group, so sometimes that may account for the fact that people who aren't trying to get noticed often do.
Other than that, I think what makes some people get noticed is that they're unique (stand out), but also that they have something particularly charismatic or appealing about their overall appearance and demeanor that appeals to others on a level that they may not even really be aware of.
In a world full of so many "screamers", the person who isn't screaming is going to be noticed. Also, though, a lot of people may wonder why it is that one person isn't "screaming" the way "everybody else" is. That, alone, may make the person just a little more interesting; but there's also a good chance people will find that person appealing - just because he's unique enough, and "himself" enough, not to be "screaming". So, even if being "interesting" isn't the appeal, sometimes being "refreshingly different" is.
OR, another possibility may be that the person is so strikingly good looking, or has some strikingly appealing feature (like really beautiful hair), people can't help but notice.
Either way, the person who gets noticed is usually both different from most other people and appealing in one or more ways.