Tick Tock Goes The Clock

The thief carefully opened the window that they had unlocked earlier that day. The mansion had sensors at every doorway, so the window was the safest way in. Added bonus that the carpet muffled their landing. The remaining sound was covered by the massive hanging clock in the main hall, its sound penetrating every room. To the residents, it was just a background sound. For any visitor, it was a nuisance.
The path to the library was easy. The building was of high quality, not a single creak was uttered by the floorboards. The library's door was easily lockpicked, just an old-fashioned mechanical lock that was more about looks than actual security. And because fire safety meant the door could be opened from the inside, the thief could lock it again and make sure nobody accidentally walked in.
It took the thief a few minutes to find the book they were looking for. As they leafed through the book, a sudden sense of discomfort snuck onto them. Something was wrong... But they could hear no-. They could no longer hear the clock.