Historically, most tombs known to have grave goods in them were robbed soon after the dead person was laid to rest. So why has the tomb that your group of adventurers broken into not already suffered this fate?
A master thief is struck by an insanity spell and thinks he is half cat. He walks around sniffing things, clawing the furniture and licking himself in the manner of a grooming cat. This is a major cause of embarassment to the guild and the master thief must be cured of his insanity.