CoS entrance in modern bathroom

[jklb66]

No: HPFGUIDX 34949

Some people have asked, "How did the entrance to the Chamber of 
Secrets, which was built 1,000 years ago, come to be in a bathroom 
with modern plumbing?"  

My theory:  You have to remember that the entrance is magic; that the 
entrance truly only comes into existence when the correct ritual 
(saying "Open" in pareltongue) is performed.  It isn't like the 
entrance is physically there, but hidden behind a secret panel.  The 
POTENTIAL for the entrance is always there instead.  Presumably, that 
room was something else at the school's founding, and was converted 
into a bathroom later.  No one found the entrance during the 
necessary renovations, because the entrance didn't physically exist.

Similarly, I bet if you took a sledgehammer to the wall at the 
entrance of Diagon Alley, you wouldn't find Daigon Alley on the other 
side.  It will only exist for you if you enter the correct way, by 
tapping your wand on certain bricks.

When the room became a bathroom, it also continued to be the room in 
which the entrance could be found.  I'm certain the Salazar Slytherin 
made sure when he cast the spell, that no matter how much the room 
changed over the years, it would continue to serve that "noble" 
purpose.  How did the snake come to be engraved on the pipe?  All 
part of the same spell cast by Salazar.  No matter what changes the 
room went through, something in the room would provide the key for 
his heir.