I lose mine all the time. You probably do too.
So here’s my idea. And I don’t even want to wait to patent it and license it… you should just build it.
It’s two credit card sized devices. One goes in my wallet. The other is taped to the back of my phone (or ipod).
Whenever they get more than thirty feet apart, they both start whistling like banshees.
Is this hard? I don’t think it is. I’d buy one. I’d even pay extra to have it built into the phone itself.