Ryuuko followed her boss's gaze and noted the... damaged wall? She wasn't an expert of architecture but the general fastidiousness this mansion exemplified from its sheer volume of maids made her expect that something like that would have been immediately repaired. Plaster was made from and could be ground back [i]into[/i] a powder, which might explain why there was something in the library... but how was it in the library and what could even have damaged the wall? 

"Maybe the damaged ofuda is because whatever the thief was doing worked imperfectly and somehow damaged the wall over in the library? That would explain the damage here and the plaster there. I cannot see how a wall would end up in the library or why that would be a thief's first choice, however."