This is what Mme Campan says in her memoirs:
dangerous character, in consequence of his disposition for intrigue.
Bell, this is very interesting! When you get into the realm of intrigue would that not be open for a lot of Possibilities.
He was a subject of the Duke of Mantua,
This is the only Prisoner Mentioned that might fit that Possibility!
Count Ercolo Antonio Matthioli: An Italian who worked for the Duke of Mantua. The French wished to buy the fortress of Casale from the duke, but Matthioli mishandled the negotiations and was imprisoned at the order of Louis XIV. Matthioli is said to have worn a mask voluntarily because it was an Italian tradition.
However, he certainly doesn't fit the role of Intrigue or dangerous.