For more information and detailed diagrams, visit http://www.arduino.cc/en/Tutorial/Tone2 today.
Get an Arduino, Breadboard, Photocell, and 8 ohm Speaker.
Ground the assembly, put in a 4.7k ohm resistor and a 100 ohm resistor, and hook the wires to the Power Supply ports on the Arduino.
Verify and upload the code to the Arduino, and change the volume and tone of the alarm sound. At http://www.arduino.cc/en/Tutorial/Tone2, you can get the code.
Hook up a Power Pack to the Arduino using a USB cable, and secure the speaker.
Hook the whole assembly, including the Arduino, Breadboard, and Power Pack, to the wall where the mirror will be hung.
Hang a mirror over the Arduino assembly, to make sure nobody can see it.
Put your special items on the back of the mirror.
Now the Hidden Place is complete. If somebody moves the mirror to get the valuables, light will hit the photocell and the speaker will make an alarm sound.