It stood there, formless, diabolical, and then she saw what was happening.

It was closing the door.
Afterward she was mercifully not to remember what came next; the figure was perhaps intent on what was going on outside, or her own movements may have been as silent as its own.

That she got into the mantel-room and even partially closed it behind her is certain, and that her description of what followed is fairly accurate is borne out by the facts as known.
The Bat was working rapidly.

She heard his quick, nervous movements; apparently he had come back for something and secured it, for now he moved again toward the door.

But he was too late; they were returning that way.