According to the book (Harry Potter and the Chamber of Secrets), the Basilisk roamed around in the castle's pipes, but the people who died or got petrified were at different parts of castle (not in pipes) when they were attacked by the snake.
Just wondering; How did the giant snake manage to pop out anywhere and attack when the only opening big enough was the one in Myrtle's bathroom.
JKR never answered that; in the books, interviews (I searched entire Accio Quote) or Pottermore for Chamber of Secrets. The most the books have is Ron/Harry's understanding that Hermione wrote the word "Pipes" meaning that Basilisk used the pipes to move around the castle.
However, "the only opening big enough was Myrtle's bathroom" isn't really a fact, it's merely the entrance to the Chamber.
The architecture of Hogwarts is shown to be variable (shifting staircases, for one; Room of Requirement for another example); so it's plausible that Salazar Slytherin, one of the 4 founders, configured the pipe system to be able to open up wherever needed - since he already "programmed" that Basilisk will travel through the pipes by creating an official Chamber entrance in the girl's bathroom. That's just my personal speculation, not canon.
The Basilisk has killed Myrtle right in the bathroom with the opening of the chamber, and has Petrified Mrs Norris close to the same bathroom.
As much as I like Colin Creevey, I believe he got Petrified when he was out in the corridors at night when students are not normally allowed to leave their common room and dormitories. This means that at that time the corridors of the castle were mostly empty, which is why the Basilisk could come out mostly unnoticed.
After Colin got petrified, the students are no longer allowed to walk in the corridors freely even during the day, they must be accompanied by a teacher. This makes the castle mostly empty except between classes, so the Basilisk could easily move on the corridors to get to the next four victims.
The basilisk was probably traveling via corridors, and only came out of the Chamber when it was safe and no students were around. It used the pipes to travel from the Chamber into the girls' bathroom (as seen in the films, they were large enough). Its whispers probably traveled through the pipes (via air) and that's why Harry could hear it speak through the walls during the plot. For other students, it either sounded like normal wind and rattles that old buildings naturally make, or maybe it was so quiet that, without understanding the meaning, it went unnoticed.
Please note that in the movies, whenever Harry found someone attacked in the corridors, there always was this great wet/slimey trail (as broad as the Basilisk) on the floor just behind the spot of attack. So the theory that it did come out of the Toilets into the corridors is verified!
