Meruin had a vacant look on her face, as if she were half-asleep. In actuallity, she had placed an illusion on herself using her grimoire and was attempting to break out of it. It was a regular kind of training for her whenever she had short amounts of free time. In the illusions she placed on herself, time moved differently; four days could have passed inside the illusion when only a few minutes went by in the real world. 

The only way to break her out of the illusion was if you entered it yourself or if Meruin succeeded in dismantling it. Or if you were Marcus just cancel it out by tapping her on the forehead.