Summary

Make your own laser motion detector. When the laser beam is broken, you can detect it and trigger some action like a bell or an email.
By using multiple mirrors, it's possible to create a security perimeter.
The laser and laser sensor are designed to fit on the same unit. The other unit is just a mirror.
The mirror is from this ebay item:
http://www.ebay.com/itm/Retro-Rose-Flower-Shape-Cosmetic-Makeup-Compact-Mirror-3D-Stereo-Double-Sided-B7-/191304244711?pt=LH_DefaultDomain_0&var=490415302758&hash=item2c8a9f1de7)
The laser is:
http://www.ebay.com/itm/Laser-head-sensor-Transmitter-Sensor-Module-for-Arduino-New-1PC-/400730216385?pt=LH_DefaultDomain_0&hash=item5d4d61d7c1
The laser sensor is:
http://www.ebay.com/itm/1-PCS-New-Laser-Sensor-Module-non-modulator-Tube-Laser-Receiver-Module-/390846857818?pt=LH_DefaultDomain_2&hash=item5b0049c25a