In the following passage from the narrative, to what mystery is Darnay alerted by his uncle's behavior?
"As he bent his head in his most courtly manner, there was a secrecy in his smiling face, and he conveyed an air of mystery to those words, which struck the eyes and ears of his nephew forcibly."
A Tale of Two Cities by Charles Dickens
In Chapter IX, "The Gorgon's Head" of Book the Second in A Tale of Two Cities by Charles Dickens, Charles Darnay, in one of his secret passages from England to France, arrives at the chateau of his uncle the marquis, the very aristocrat who has run over the peasant child with his carriage. As they sit down together to supper, Charles tells his uncle that the family name is anathema to him and it is detested throughout France. For this reason, he informs his uncle, he has changed his name and works in England. Darnay renounces any claim to the family property, as well. He also acknowledges that he is aware that his uncle's "diplomacy would stop me by any means, and would know no scruple as to means" in his purposes, but the uncle is out of favor with the court. In other words, Darnay know that the Marquis does not approve of his working to enable the peasants to suffer less and that he would have Darnay put in prison if he were not in disgrace at court.
As he continues to tell his uncle that he cannot look at the faces that he does not see "the dark deference of fear and slavery" upon them, the Marquis cynically replies,
"The dark deference of fear and slavery, my friend,...will keep the dogs obedient to the whip, as long as this roof...shuts out the sky."
Clearly, there is tension between Charles and his uncle. Charles tells his uncle that his mother has implored him to "have mercy and to redress" the wrongs upon the peasants, but the Marquis, with situational irony, vows that he will die, "perpetuating the system under which I have lived." Then, when Charles says that there is a curse on French land, the uncle asks him how he will live. In England, Charles informs him, he has been working.
"They say, those boastful English, that it is the Refuge of many. You know a compatriot who has found a Refuge there? A Doctor?"
"With a daughter?"
"Yest" said the Marquis. "You are fatigued. Good night."
As he bent his head in his most courtly manner, there was a secrecy in his smiling face,...
The Doctor and the daughter alluded to, of course, are Dr. Manette and Lucie Manette. The mystery here is their connection to the family of Charles Darnay. Perhaps, Darnay senses something as he must recall that Dr. Manette has stopped Darnay from revealing himself to him. This mystery is later solved when the letter of Dr. Manette, the prisoner of North Tower of the Bastille, is discovered by Ernest Defarge.