When did they become invisible?
In the first Harry Potter movie, does no one in a busy train station notice groups of people running and disappearing into a wall? This is when the newbies (Ron and Harry) run into the wall for the very first time to catch the Hogwarts train.
When did they disappear?
You are bringing up an interesting aspect of Muggle / Magic interaction. There are aspects to a magical person’s awareness that lets them see things that Muggles do not (or cannot). The entrance to Platform 9-3/4 is one of them. The entrance to Diagon Alley is another. The phone booth entrance to the Ministry of Magic is yet another. One second a magical person is there. The next second they “are not”, and they go unnoticed by Muggles.
In my opinion, they are not actually gone. They have entered into what a Muggle would call a distorted part of their surroundings that are no longer linearly three dimensional. Platform 9-3/4 is there; it’s just not perceived in the normal sense by Muggles. There are other examples of where three dimensional space – taken as given by Muggles – are distorted to the advantage of magical people. The Knight Bus is an example (where it can get narrower to avoid a collision). The headquarters of the Order of the Pheonix is another. The Weasley tent at the Quidditch World Cup is yet another.
They “disappear” when the magical person enters the no-longer-linear space. To a Muggle, who sees only linear space, they “disappear”. To the magical person, who has the ability to enter into the non-linear realm, they either seem to enter a place that looks normal, or least tolerable (“Fellas, why the long faces? [on the Knight Bus]).
I am sure there are other examples.
Pictures about "When did they become invisible?"
When did the Invisible Man become visible?Answer: The invisible man (Griffin) first became visible after he slipped into a big London store for keeping warm and overslept there while wearing some clothes taken from the store. The clothes made him visible to the shop assistants when the shop opened in the morning.
Why and how did he become invisible?Solution : Griffin was a brilliant scientist. He had carried out several experiments to prove that the human body could become invisible. At last he swallowed certain rare drugs and his body became as transparent as a sheet of glass.
Why did the Invisible Man become invisible?In the original work, Griffin is a scientist whose research in optics and experiments into changing the human body's refractive index to that of air results in him becoming invisible. After becoming invisible, he wraps his head in bandages and dons a pair of goggles or glasses in order to enable others to see him.
How did the first Invisible Man become visible?The invisible first became visible when he stepped into some mud and left footprints as he walked.
What If You Become Invisible? | Invisibility | The Dr.Binocs Show | Peekaboo Kidz
More answers regarding when did they become invisible?
If muggles were to watch a wizard disappearing at a stone wall, they would for sure be astonished (at least).
But there is a guard at the train station that takes care, that muggles don't see that happening (and probably he is able to do the obliviate- spell, but this is never stated anywhere).
You can read about the guard in Harry Potter and the Philosopher's Stone:
It took quite a while for them all to get off the platform. A wizened old guard was up by the ticket barrier, letting them go through the gate in twos and threes so they didn’t attract attention by all bursting out of a solid wall at once and alarming the Muggles.
*Harry Potter and the Philosopher's Stone, Chapter 17 (The Man with Two Faces)
Sources: Stack Exchange - This article follows the attribution requirements of Stack Exchange and is licensed under CC BY-SA 3.0.
Images: NOHK, Harrison Haines, Cup of Couple, Brett Sayles