So here is the question, is it physically possible for a "human" (human size object) to move in a CONFINED environment (lets say, 20 m×20 m×10 m ) so fast that a regular person would be unable to SEE? If it is possible, how fast must this "person" be?
The human eye relies on reception of photons to "see" something. There is no speed below the speed of light where an object will not reflect or emit photons. Speed has nothing to do with reflection and emission, except as you approach the speed of light, which is far above the speed a biological creature can attain under their own power.
So to answer your question technically, no, there is no speed where an object will cease to reflect or emit light in the conditions you've described.
However, you might consider perception as part of "seeing." The human eye is analog, and the data is being processed by an analog brain. The receptors measure the amount of light received and send signals - more if more light is present, fewer if less light is being received.
Further, being biological, there's an amount of delay involved. You can turn on a light, and it won't be sensed instantly once photons start hitting the receptors, instead the receptors will ramp up their signal output, and when the light ceases, they will relatively slowly cease signalling to the brain.
It's this property that makes it appear that static images shown in a sequence of 25 frames per second or more are not indistinguishable as separate frames, but as video. In essence, the receptors act as low pass filters.
So a sufficiently fast moving object might appear blurred to the human eye.
However, humans are able to perform Smooth Pursuit, where the eye can follow a target which will avoid the blur, and the consequences of Saccade don't cause the visible scene to disappear.
Smooth pursuit occurs up to 30 degrees per second, but this is a maximum in one direction. The human eye moves more slowly when trying to go back and forth following an object in a small area of their visual range.
Paint a circle on a ceiling fan. At the low and medium speeds you should be able to follow it with your eyes several feet away. At high speed you should find that it starts to blur or lengthen, or that your eyes can't keep up and you start following it via saccade movements.
Now do the same thing with a desk fan - at most speeds you cannot follow it at all, and, in fact, the blades become transparent. They aren't invisible, but the light reflecting off them provides the perception of invisibility, primarily because you can see objects behind them which should be obscured by them.
You can move your hands fast enough that your eyes cannot follow them using smooth pursuit, at arms length. Focus on an object beyond your hands and as you move them quickly past your vision you will see just a blur.
At 20 meters from the target, there are humans capable of moving their hands quickly enough that you would see little more than a blur, and you would be able to see objects behind the area their hands are operating in.
However they would still be perceived, and you would know that their hands occupy that space. So depending on what you mean by "see" the answer is either, "No, they will always be seen," or "Yes, it is possible to move a human hand fast enough within your field of view that you wouldn't perceive the hand, but only the blur, and it would never fully block your view to objects beyond it."