In Season 1, Episode 6, the one with the surgeons entering the mint thinking they're gonna somehow find the thieves and hostages. Right after they ran out they ran to the Inspector General saying that "They've changed masks, it's a slaughterhouse". Are they implying that if they did have the old masks, they would be able to tell the thieves from the hostages?
There policemen trying to infiltrate the group by going in wearing the same outfit and masks - they would blend in and no one would know that they entered the mint. When the gang and hostages changed their masks to a different one, it destroyed the infiltration plan. The policemen were wearing old masks and would not blend in.