"Well I remember hearing about this theft prevention system I think if you get too far from your cell phone or something like that, this thing on your key ring starts beeping or vibrating."
It's called zomm. (www.zomm.com
) It will only work with bluetooth devices. And if your cell signal breaks, as in if your phone is turned off or drops a signal deep playa, the alarm goes off. I have one. It works quite well but I've had to let my co-workers know how to turn it off becuase I'm always heading off at work to a meeting with my cell phone and leaving my car keys behind. Doh!
You're better off looking into one of the aforementioned devices.
We build baskets on our bikes that will fit our backpacks and then locks so they don't wander off without us.