The Contessa wrote to Corbario two days later, addressing her letter to Rome, as she did not know where he was.

It was not like her to meddle in the affairs of other people, or to give advice, but this was a special case, and she felt that something must be done to save Marcello; for she was a woman of the world, with much experience and few illusions, and she understood at a glance what was happening to her dead friend's son.
She wrote to Folco, telling him of the accidental meeting in the portico of the Theatre Francais, describing Marcello's looks, and saying pretty clearly what she thought of the extremely handsome young woman who was with him.
Now Paris is a big city, and it chanced that Corbario himself was there at that very time.

Possibly he had kept out of Marcello's way for some reason of his own, but he had really not known that the Contessa was there.

Her letter was forwarded from Rome and reached him four days after it was written.

He read it carefully, tore it into several dozen little bits, looked at his watch, and went at once to the quiet hotel in the Rue Saint Honore.