Can a speedster move fast enough in a room to be unnoticed by people in it?

It depends on whether you are watching the televised Flash or the comic Flash to determine the special effects of the Flash's Speed Force-derived abilities. Each has different requirements based on the media and as such will appear differently.

In the Televised Series

• In the modern Flash depiction, he is shown having the ability to move at speeds beyond human visual awareness. He can appear and disappear in the blink of an eye. Timed properly, he can move past someone and they never know he was there.

• At his top speeds, no one will notice him except by the very limited atmospheric effects he creates as a side effect of his speed. I say limited because if true physics were in action, he would make much more of a physical disturbance.

• Given real world physics, the Flash should have quite the atmospheric effect, particularly when he reaches the speed of sound, but thanks to the physics distortion built into the Speed Force, he does not generate the powerful bow shock and sonic disturbances associated with his rate of speed. Nor does he disturb the air around him at anywhere near the levels he should.

• The question says can he move without being seen? Yes. We have seen him move without generating any special effects. Normally the Flash does not generate the lightning effects unless he wants to. We know he can control their visibility because he uses his lightning to obscure the vision of two law breakers (when he had lost his powers and was using holograms to simulate the Flash still being active.)

Zoom, on the other hand does not suppress his powers, using the blue lightning around him as a sign of his power and his greater speed to intimidate others.

Can he move through a scene without being seen or noticed? It should be possible, otherwise scenes, such as the episode Invincible, where he is talking to his boss as the Flash, zips out and then suddenly reappears as Barry (without the corresponding swooshing and lightning) would not be possible. Strangely enough, most of the time it is the leaving of the scene where papers fly about, not the arrivals. Both should have the same amount of atmospheric disturbance.

• Because television is a visual medium, the tell-tale swooshing sound effect, the yellow lightning and the flying papers are all signs that the Flash has entered or left the scene. This is done for the viewer visual edification, showing how fast the Flash appears to be; a form of eye candy for the viewer.

• This eye candy as I call it can be traced back to the comic depictions made famous by Carmine Infantimo who is the iconic artist who produced the most dynamic depictions of the Flash using his powers during the Silver Age.

In the Comics

In the comics, the Flash has show he has complete control of his atomic structure. So much so he can manipulate his molecules for a number of feats the televised Flash is still learning to do.

• He can vibrate and disassociate his molecular structure such that the electromagnetic shells around his atoms no longer interact with normal matter. He is out of phase and can pass through ordinary matter with precise control.

• He is also capable of vibrating his molecules so that light passes completely through him, rendering him invisible to normal senses. Superhuman senses or other speedsters who may have familiarity with this type of vibration-manipulation based invisibility can become aware of him.

• Most incarnations of the Flash and other speedsters have far greater control over their Speed Force interactions such that they do not disturb anything in the environment, unless they make a conscious effort to do so. This is how Barry can use sonic booms as a weapon, either as superfast finger-pops or by moving faster than sound and targeting the bow-shockwave he generates to stun opponents.

• The Flash has had these powers most of his career with his abilities growing more powerful over time. At one point the fastest he could move was the speed of light. (Or so said the Handbook of the DC Universe.)