In the Prefect's bath.

Hermione had had a stressful day of classes and performing duties of Prefects. So being one of the latter, she decided to use the baths that night after dinner to ease the stress. After she stripped herself in her dorm and donned a bathrobe, she gathered her personal belongings in a basket and headed out the door. As she did so, her wand slipped out of the basket and rolled across the floor.

When she entered the bath, she locked the door behind her and proceeded to fill the pool within. She then set her belongings alongside the pool, removed her robe and hung it along with the towel she brought along. When the bath finished filling, she slipped her naked body beneath it's bubbly embrace and felt all of her cares and stresses melt away.

It is just unfortunate that she was to become stressed again after her relaxing bath, she would be in a situation even she didn't anticipate.