She wet the bed, didn't she. That's why the futon "mysteriously" dissappeared... The least you could do is replace what you've taken out of shame...
On a side note, the social order in this school for nobles seems awfully lax. I mean sure a student sneaking into the opposite genders room might go unnoticed once or twice, but this noble girlie is hauling futons in and out of a boys room. You would think an adult would have commented on it by now.