The Fourth Floor WAS (Dumbledore in CoS was Re: The Keeper of the Keys.

[M.Clifford]

No: HPFGUIDX 113530

> > Valky:
> > At the very least he created a secret entrance 
> > to Hogwarts of his own via the fourth floor secret passage to 
> > Hogsmeade. 
> > 
> 
> Geoff:
> Can you enlarge on your source material for suggesting that?
> 

Valky:
Why, certainly Geoff.
In POA when Harry is introduced to the Marauders map by Fred and 
George they inform him that the secret passage on the Fourth Floor 
that leads to Hogsmeade suddenly and by no reason apparent to them 
became caved in and blocked during the winter of the previous year.
The Winter of the previous year was of course Christmas of the year 
COS a time when "Harry, Hermione and the Weasleys had the run of 
Gryffindor Tower....." because noone else stayed.

Between Gryff Tower and the Fourth floor are the Sixth which we know 
nothing about and quite possibly is passed over by the staircases 
because no canon ever places Harry there in five years, and the 
Fifth which is the prefects bathroom floor that Harry discovers, to 
his surprise, exists after he is told about it. 

Suffice to say, it appears somewhat that the Gryffindor common room 
might lead out to a staircase usually facilitating the lower 
grade/Non prefect students direct access from the GT entrance to the 
Fourth Floor and bypassing the two between floors. 

So to say that if Tom!Ginny was having the run of Gryff Tower with 
her unsuspecting brothers in the winter of COS, the opportunity for 
Tom to become easily acquainted with the relative immediate 
surrounds of Gryff Tower is likely to have been _in the Winter of 
COS_. 

So it flows like this: 

Before Christmas, Ginny gets private tour of GT from F and G they 
tell her, and so likeise Tom, about the secret passage, later 
dismissing it from their minds. 

After Christmas, while HRH are busy with their polyjuice potion and 
the ensuing events, Tom!Ginny find themselves some bit more alone 
than usual and Tom takes advantage of the quiet moment to slip down 
to the mirror and use the knowledge he has gained. Or even during 
the Christmas feast where I simply cannot find Ginny anywhere.

Either way the unexplained collapse of the fourth floor passage in 
winter of COS and the lack of Tom!Ginny having anything satisfying 
to do at that particular time leads well into suspicion dont you 
agree? And this is my theory.