When she was half way up the stairs she heard somebody moving about, but it was not like Mabel's step.

The movements seemed cautious, and there was something awkward about them.

Agatha, who wore felt-soled slippers, stopped and listened, while her heart beat fast.
She heard nothing now, but felt alarmed, and wondered what she ought to do.

A call would probably bring an answer that would banish her fears; but suppose it was not Mabel she had heard?
There was, however, another way of finding out, and with something of an effort she went upstairs.
Mrs.Farnam's room was on the landing, and Agatha turned the handle cautiously.

The door would not open, and it was obvious that Mabel had locked herself in.