When I imagine no-clipping, I just think of falling through a wall. Done in less than 5 words. But to really define my view of the subject, I have to get into more detail.
Instead of just "a wall that isn't there", I think of it more like a portal. You can't phase in and out, as soon as you hit that wall, you're out like a light and just as soon you're back. But somewhere else. In the speedrunning definition of no-clipping, you can't just come back the way you came. Another piece to put into play is the fact that these usually happen on unbalanced and tilted areas, mostly corners. I don't think you could just fall. I think it would take being forcefully thrown into a corner or tilted plane of some sort and blacking out for less than a second, to wake up here, in The Backrooms.
From the view of witnesses, it would appear as the person that no-clipped was "glitching", falling straight through but not. If the witness were to touch the area the person no-clipped into, it would remain solid, as I believe no-clips would only take place when reality is bugged for a split second, with a person destined to fall straight into that bug and out the other end. This scenario, to have these astronomically inconsistent events happen at the same time and place is a coincidence, but also "a fate foretold in the stars".
Welp. That's my take on the matter. Great question, Robert! Side note: Is there an official spelling? "Noclip" or "No-clip"? Have a nice life. -Iamalemon