He would be the first.

The Cardinal would be told that Paolo had not slept at home, in his lodging high up in the old palace, and he would send at once to Marzio's house to know where his secretary was.

Well, he might send, Marzio would answer that he did not know, and the matter would end there.
It would be hard to sit calmly at the bench all day with Gianbattista at his side.

He would probably look very often at the iron-bound box.
Gianbattista would notice that, and in time he would grow curious, and perhaps explore the cellar.

It would be a miserable ending to such a drama to betray himself by his own weakness after it was all done, and Paolo was gone for ever--a termination unworthy of Marzio, the strong-minded freethinker.