Her robe and towel are taken.

The bubble bath so soothed Hermione that she soon fell asleep. It was just her bad luck that her robe and towel fell onto the floor, for an inexperienced house elf working late in the castle took this as a signal to to pick up any dirty towels left in the bath and take to the laundry. So this elf appeared, picked up the towel and robe, and disappeared without ever noticing the brown bushy-haired student sleeping in the bath.

When Hermione finally woke up, it was dark outside the castle. Noticing this and that she was out way past the school's curfew, she looked around before climbing out with the intention of drying herself magically and returning to the Gryffindor tower as fast as possible. It was then that she realized her wand was missing. On top of his, she noticed her clothes were missing. After looking around and finding them nowhere in sight, Hermione took some to contemplate her predicament. She was naked, partially wet (by now), and had to get into her dormitory on the other side of the castle. Seeing the darkness of the night through the windows, she figured she would have some time to sneak through the castle without being spotted by anyone ( she would have to keep clear of the cat though). So she unlocked the doorway to the hallway and, with one arm covering her breasts and the other hand grasping her shower basket which served as cover for her other parts, crept outside. It was just after exiting the Prefect's Bath and closing the door behind her that Hermione discovered another problem.