The defences weren't triggered, the exterior was an impenetrable wall... and to have triggered the library ward after activating the defences would have meant resetting them. Presumably, then, someone who could have pulled off hiding their tracks that well would have fixed the library barrier before even getting around to this one. Or was it an unrelated incident, or even something meant to throw them off their trail?

"Which wall is the exterior?" the Yakuza heir questioned, stepping forwards into the corridor and looking for anything that seemed out of place in a more mundane manner. She might not have the skill to detect magical anomalies yet, but somewhere more damaged or disturbed than another point, or a region strangely free of dust if the place was regularly cleaned... she could look at that. Though with no trace at all, another possibility came to mind... "Might we be looking at a 'phantom thief'? Could someone have magic to pass through physical barriers undetected, or could ghosts do so?"