He had forbidden her to go near it.

She turned and looked at him.
"Found you out! How ?" she asked, with a queer smile.
"Saw through my disguise." "Yes--she would do that!" said Etta aloud to herself.
"What is this door ?" she asked, after a pause.
"It leads to an inner room," replied Paul, "where Steinmetz usually works." He passed in front of her and opened the door.

As he was doing so Etta went on in the train of her thoughts: "So Catrina knows ?" "Yes." "And no one else ?" Paul made no answer; for he had passed on into the smaller room, where Steinmetz was seated at a writing-table.
"Except, of course, Herr Steinmetz ?" Etta went on interrogatively.
"Madame," said the German, looking up with his pleasant smile, "I know _every thing_." And he went on writing..