After you get to the main house and watch the dinner scene, you'll shortly arrive at the first safe room. Once you get to this safe room, head towards the door, but before you leave, look to your right and on the metal shelf will be an orange-red tin box. Look inside this box and you will find a lockpick. Take the lockpick (this unlocks the "A-ha!" achievement, if you don't already have it). Exit the safe room and immediately turn right. Walk all the way down the hall. You'll notice two drawers here. One is next to the boarded-up window, and the other drawer will be next to a door. Go to the drawer closest to the door, and press A when looking at the top shelf of this drawer. Your inventory will pop up, prompting you to use an item. Select the lockpick, and the achievement will unlock.