<Spoilers ahead! Don't read if you want to find it on your own!>
There's a hidden QR code only visible from a specific angle inside the secret room!
Prerequisites: You must have finished the main campaign, making the door with the "open" sign accessible.
How to reveal it: 1. Put your back to the safe. 2. Swivel the camera so you can see the back of it. 3. Carefully walk away from it to reveal a hidden QR code for a telephone number (long distance charges may apply). 4. Scan it with your phone using a bar or QR code reader.
Dialing this number will result in a prerecorded message that asks you for your e-mail address and other contact info, to which the voice promises to send a little something to.