See if any one is on that landing.

I fancied that the door opened." The maid shook her head incredulously.
"I don't think so, my lady," she said.

"No one but the waiter and the chambermaid who comes in to clean the apartment, ever comes that way." She crossed the room and tried the handle.

Then she turned towards her mistress in triumph.
"It is locked, my lady," she reported.
Ella rose to her feet and herself tried the handle.

It was as the maid had reported.