When investigating an art heist, history has shown that a close look at employees is, unfortunately, necessary. The FBI estimates that around 90% of major art thefts have included the work of an insider. In fact the Bureau has produced a valuable document for security personnel in all sectors to help protect employers from insider theft.
It's important to note that in museum thefts, the involvement of an insider might not always mean direct complicity. One important message that security chiefs must communicate to employees is the World War II adage that is as true now as it was then: Loose Lips Sink Ships. When employees are incautious about what they view as lax security at their institution, they risk sharing vulnerabilities with potential criminals.
Of course, there are countless cases of high-value heists perpetrated by trusted employees themselves, which is one of the reasons that many museums require employees to undergo bag checks when they leave their workplace. Security personnel cannot be exempted, as they typically have more access at the most sensitive hours and have been found to have been behind--if not directly responsible for--major heists.
Such seems to be the case with the December 2017 theft of a 16-carat diamond from James Stunt, who was married to Formula 1 heiress Petra Ecclestone, as part of an alleged £20 million theft from the art dealer’s mansion in Belgravia. John Simpson, a crime correspondent for the Sunday Times, has written about the crime and the allegation that Ecclestone's ex-husband was victimized by his bodyguard, 34 year old Justinas Ivaskevicius, The bodyguard has denied the charges. More on the case can also be found in the Telegraph here.