While not a legal requirement, alerting the police would ordinarily be standard procedure for luxury hotels such as the Carlton.
The CGT union said Monday that the Carlton had already been the victim of a jewellery heist in 1994, when a security guard was seriously wounded by a bullet.
According to a source close to the case, the jewellery house Leviev was using its own security guards for the ill fated “Extraordinary Diamonds” exhibition, which opened on July 20.
Cannes has fallen prey to thieves several times recently, notably during this year’s film festival, which attracts a glittering array of celebrities from the movie world.
In a pre dawn heist at a hotel during the festival in May, thieves stole jewellery worth $1.4 million due to be loaned to movie stars.
That robbery took place in the hotel room of an American employee of Swiss jeweller Chopard while she was out for the evening, police said.
In a scene straight from a Hollywood film, a strongbox containing jewels was ripped out of the wardrobe and carried off, they said.
In a second theft during the festival, robbers made off with a diamond necklace with an estimated value of $1.9 million.
At least two apartments rented by film executives were also burgled during the 2013 festival, with thieves taking cash, jewellery and other personal items.