Why is Petra nervous in Delia Dell Hall?

Petra and Calder decide to look in Delia Dell Hall for the painting. Petra grows nervous as they wander the beautiful and unique building. Petra notices the windows and wood resemble Vermeer paintings. Other aspects of the place resemble words and ideas Mrs. Sharpe had come up with when playing with the pentominoes. She feels sick from all the coincidences and they hurry out of the building.