The key was in the door.

He turned it and put it in his pocket, shot the heavy bolts, and ran downstairs.

When he got to the bottom, he tried the door of the major's quarters.

It was unbolted, and he felt absolutely certain that the major would be out as, with the other officers, he would have gone down to the gate to receive those of the incoming detachment.
On opening the door, he saw the articles of which he was in search--a long cloak and a regimental cap.

These he at once put on.
After a further search, he found a pair of military pantaloons and a patrol jacket.