Those Pesky Metal Detectors
This was done by me and a few of my buddies at the local university, and it all happened by chance.
While walking past the metal detectors (illegal book detectors? what the hell do you call 'em?), wearing our headphones, we heard a high rining noise from our headphones. The noise grew as we got closer to the detector, and softer as we were farther away.
So who gives a rats behind? Well, for one, you test to see which detectors are working, and which ones are not. Use your imagination to figure out the rest.
I in now way condone stealing books from the library - because you can always steal it offa the 'net :p If anyone knows more about this topic, or detecting security devices (in general), any follow ups would be great!