Many people use their keychain to flash their lights and locate their car in a parking lot. If the signal went 2-ways, you could also use your car to locate your keys. (Some people lose their keys all the time!) The signal could originate from a keypad on the car door, and broadcast, for example, from the garage into the house and under the bedroom dresser where the keys had fallen. The chirping keychain could then be recovered by the relieved individual.
IDEATION: April 8, 2006