I saw this device in a store last week. It's labeled as a wireless keyfinder. There are two components to make it work. One is a key chain with a chip so it can be detected electronically. The other one is the detector. You press the button and it will make the keychain attached to the missing keys emit a sound and blink (I think) which will make the keys move apparent as to where they were left.

I was impressed with the gadget... for about 30 seconds... until I thought of this scenario...

"What if the keyfinder is missing instead of the keys ?"

And

"If you lose the keyfinder, but not the key chain with the keys, you can still survive because the important thing is you have your keys with you, demoting this gadget into a mere key chain."

So lesson is...

Don't buy this. Just...
ALWAYS REMEMBER WHERE YOUR KEYS ARE.