X stays still, listening for the sound of another door opening.

It doesn't come.

The muttering gets closer.

X does not sigh.

She does, however, peer over the railing, making note of the way the guard is looking at his feet.

This will simplify things.

Then she drops down on him.

Carefully.

He does not see it coming.

But it does mean she'll have to work faster. Someone will notice that he's missing before long.

(It might be useful.)