I wonder if RFID would be an effective solution here. They're cheap, small, and don't require batteries, so all you'd need to do is make a device that can gener

I wonder if RFID would be an effective solution here. They're cheap, small, and don't require batteries, so all you'd need to do is make a device that can generate a powerful, directional signal and you could quickly locate whatever you're looking for. I'm not an expert on RFID, but it should be a workable solution, and would work even if the device is off (e.g. ran out of battery).