What mystery is introduced at the end of chapter 5? (Book 3 Chapters 1-7)

The mystery introduced at the end of Chapter Five was the identity of the prisoner summoned to appear in front of the Concierge.

Who could that be with Mr. Lorry--the owner of the riding-coat upon the chair--who must not be seen? From whom newly arrived, did he come out, agitated and surprised, to take his favourite in his arms? To whom did he appear to repeat her faltering words, when, raising his voice and turning his head towards the door of the room from which he had issued, he said: "Removed to the Conciergerie, and summoned for to-morrow?"