My first instinct was to laugh... But seriously -this sort of behavious jeopardizes the availability of lenses for handling at trade shows and should be deplored.
Now that's out of the way, Sigma state that there were two pre-production lenses in existence; which mount was the one that was stolen? If it was Sigma SA mount, then it shouldn't be too hard to find the culprit; I would get the police to interview both suspects.