sensor - technology to search small objects like pendrive, key-chains etc

I am a web developer by profession. few days back I lost 8gb pen-drive :( I don't it is in home or somewhere else.... So now I am thinking of a device which can be attached with small objects so that their location can be detected ...like

Clap of sound Amplitude modulator transmitter attached to pen drive which can transmit special signals Or any Bluetooth detector ....which can be used from mobile to detect

any thing I don't know

It should be effective , energy efficient and small

Can you suggest any thing :/

I was searching for sound and AM transmitter ....