Unsuccessful in his attempts, he finally straightened up with an exclamation of anger and disgust.

Going over to a small cupboard built in the wall, he opened it, and, stooping, pressed his finger against some hidden spring.

Then the wall opened and the light from the lantern disclosed an inside recess.
Lifting the box, he carried it over and deposited it in the opening, and at his touch the panel slid back into place.

Quickly locking the cupboard, he placed the key in his pocket, and, extinguishing the lantern, strode towards the door.
Once outside, he passed so close to Grace that by stretching out her hand she might easily have touched him, as she lay flat on the rain-soaked ground, scarcely daring to breathe.
The stranger paused to lock the door, and Grace heard him mutter: "Nice night to send a pal out in, and on a still hunt, too.

Nothing short of soup'll open up that claim.