A guy who works nearby just stopped in to see if anyone found his car keys. Nope. But it occurred to me that if you didn't know where your car was in the mall parking lot, you could press a button on your car key and make your car beep. Why hasn't anyone invented a beeping keyring? Maybe something that works via Bluetooth so you can tap a button on your phone and make your keys beep.
Of course, if you don't know where your phone is, and you don't have a web browser handy so you can fire up the Find My iPhone tool (or Android equivalent, or whatever), but you do have your keyring, you could press a button on the keyring and make the phone beep.