What technology would I need to alert a gadget of something specific in its vicinity?

I am trying to work out wjat type of technology I would need to send a signal (warning) that a specific item is in range of an item say 5 meters. Like a transmitter and receiver type devise. Any ideas?What technology would I need to alert a gadget of something specific in its vicinity?
1) video capture. If the picture changes, alert the device.

2) infrared. send out a pulse of infrared light. wait for it's reflection. Measure the time delay and divide by 2 to get distance.

3) proximity (capacitive). As the dielectric changes due to foreign objects between / near two plates, the capacitance changes.

4) ultrasonic. just like infrared, but uses sound too high to hear.

5) RF. radiate a radio signal. Watch for its reflection from metal objects.

What technology would I need to alert a gadget of something specific in its vicinity?
ya this is a dual stage motion sensor for car alarms but it has standard 12 volt negative outputs for each zone



http://cgi.ebay.com/DEI-508D-DUAL-ZONE-M鈥?/a>





this is a link to a e bay ad but you can find this at any local car stereo shop the dual stage ones work better than the singal stageWhat technology would I need to alert a gadget of something specific in its vicinity?
Now a days most cars(modern) are using a technology,which alerts with a high sonic whisling sound when ever some one passes near by.Please contact a workshop for information.
It depends on the properties of the item, body warmth for example.