Ginny crept along the unmarked tunnel in near-darkness, trying to make
sense of where its twists and turns might be leading her.  Not knowing
where it might dump her out -- or who else might have access to it --
she kept the light at her wand-tip to the faintest possible glow, barely
sufficient to light her way.  Not that it made so much difference:  by
preference she navigated mostly by touch, or by muscle memory if she 
knew where she was going.

She wished, not for the first time, that Harry had thought to leave
the Marauder's Map behind once he knew he wouldn't be coming back to
Hogwarts this year.  They could certainly have used it in his absence.
Instead, Ginny snuck out of her dormitory most nights to find, map,
and mark as many of Hogwarts's hidden places and secret passages as
she could find -- some on tips her brothers had given her over summer
hols, others she'd worked out on her own.  This one, starting at a
bit of inconspicuous grill-work in the Transfigurations classroom,
was one she'd thought to explore on her own.  It seemed to be meant
for ventilation:  she'd passed several more grill-covered openings
that looked out onto classrooms, and there seemed to be a faint
breeze from somewhere up ahead.

She rounded a corner to find that the shaft took an abrupt turn --
straight up.  The breeze here seemed a bit stronger, and when Ginny
looked up she thought she could detect the faint silvery glow of
moonlight somewhere far overhead.  There were no other branches
except the way she'd already come; so Ginny secured her wand into
a little holster at her belt, leaving its glowing tip exposed, and
started to climb.  The wall here felt like rough-hewn stone, not
unlike the exterior of the castle, and it was not hard to find
hand- and footholds.  Ginny moved quickly but quietly upward,
keeping her eyes open for anything interesting.