The House of the Schemers by Fred M. White

Quite suddenly a light had gleamed in the window of the locked room adjoining Lady Altamont's boudoir. It was a brilliant light, and cast two shadows, clear cut as crystal, on the blind. First came the outline of a man smoking a cigarette. He held in his hand a bottle, obviously containing champagne or some other sparkling wine, for he could be seen to cut the wires and expel the cork. It seemed to the girls watching him from below that they could hear the pop of the cork as it left the bottle.

"Are you sure that is the room leading from the boudoir?" Grace asked.