In Charles Dickens' novel, A Tale of Two Cities, Book Two, Chapter 14, entitled, "The Honest Tradesman" is particularly ironic, but it is not until one reaches the end of the chapter than one can be sure why a grave might be missing a body.

"The Honest Tradesman" is about Jerry Cruncher. In eNotes.com's character descriptions:

Jerry Cruncher...[is] an employee at the London banking house of Tellson and Company by day, [and] a resurrection man (grave robber) by night.

The story's text gives the reader the answer listening to Jerry, Sr. and his son, Jerry.

“Father,” said Young Jerry as they walked along: taking care to keep at arm’s length‚ and to have the stool well between them: “what’s a Resurrection Man?”

Jerry, the father, asks the boy how he (the father) should know that. Jerry who witnessed most of his father's "business" the night before, shrewdly responds that he thought he father knew everything. Placated in that his son has such noble thoughts of his sire, the father responds that a Resurrection Man is a tradesman. Son Jerry asks his father what kind of goods this tradesman works with.

The reply:

'His goods,' said Mr. Cruncher, after turning it over in his mind, 'is a branch of Scientific goods.'

'Persons’ bodies, ain’t it, father?' asked the lively boy.

'I believe it is something of that sort,' said Mr. Cruncher.

'Oh, father, I should so like to be a Resurrection Man when I’m quite growed up!'

Hearing this, Jerry, the father, is quite pleased with his son. It's as if the boy has said, "I want to be like my dad when I grow up!" Jerry advises his son: "Be careful to dewelop your talents" as well as who he talks to, and he might just be able to do be a Resurrection Man someday. And...

Mr. Cruncher added to himself: 'Jerry, you honest tradesman, there’s hopes wot that boy will yet be a blessing to you, and a recompense to you for his mother!'

In essence, this "honest" tradesman works for the bank during the day, but steals bodies at night. The fact that they are for "Scientific" purposes leads one to believe he sells them to doctors for research which would have been a crime in the eyes of the Roman Catholic Church. The entire chapter is rife with irony as he belittles his wife for her concerns, mocking her religious beliefs, when it seems he has no qualms about disturbing and stealing bodies buried in holy, consecrated ground. And his "dishonesty" is something he wishes on his son!

The grave is empty because a grave robber (like Jerry Cruncher) has stolen the body and sold it.