Some thieves operate on an opportunity basis, striking when a small chance presents itself. 

Something as simple as a front door being left unlocked can allow for an uninvited visitor to slip in undetected and steal a small item such as car keys, wallet or mobile phone whilst the homeowner is upstairs or in another room.

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Add a Magnetic Sensor to the door and you can be notified when someone opens the front door by a different ring tone.