It sucks that this happened, but I heard of this happening years ago. Someone locked up a bike, and every day, another piece was missing, until all that was left was the frame. Then he went back and unlocked the frame and took it away. It might have been a social science experiment. Still, sorry about your bike.

Rule of thumb is to always secure your bike with a minimum of 10-15% total cost of bike in locks. You also want at least three to properly secure any easily removed parts (handlebars, seat, quick release tires, etc) to the frame. Unfortunately, just securing the frame isn't enough. :(