The following websites offer databases for registering and searching for stolen musical instruments. In the event you suffer the loss of a musical instrument, contact the appropriate law enforcement authorities first and then consider contacting all local music stores and pawn shops with serial numbers and descriptions.
RFID's - Retrofitting Instruments With ID Chips: These are available @ Acoustic Music.Org
RFID (Radio Frequency Identification) tags are tiny microchips, some only 1/3 of a millimeter across. They act as transponders, always listening for a radio signal sent by RFID readers. When the RFID receives a particular radio query, it responds by transmitting its unique ID code, perhaps a 128-bit number, back to the reader. Most RFID tags do not need batteries: they are powered by the radio signal that wakes them up and requests an answer. Most of these "broadcasts" are designed to be read from a very short distance: a few inches to several feet, depending on the size of the antenna and the power driving the reader. RFID chips cost very little and the size allows them to be retrofitted without damage to musical instruments. The following are companies offering chips for musical instruments.
Snagg - RFID chips for musical instruments - Available @ Acoustic Music.Org