In A Tale of Two Cities, what type of business did Miss Manette testify that the prisoner was involved in?

In Volume II, Chapter 3, when Miss Manette testifies about the prisoner's line of work, this is what she says:

"He told me that he was travelling on business of a delicate and difficult nature, which might get people into trouble, and that he was therefore travelling under an assumed name."

She goes on to explain that the prisoner's business requires him to travel often between France and England.

So, although she doesn't say it...

In Volume II, Chapter 3, when Miss Manette testifies about the prisoner's line of work, this is what she says:

"He told me that he was travelling on business of a delicate and difficult nature, which might get people into trouble, and that he was therefore travelling under an assumed name."

She goes on to explain that the prisoner's business requires him to travel often between France and England.

So, although she doesn't say it directly, Miss Manette's testimony hints that the prisoner was a spy. By saying that he traveled under a fake name, that his work was secret and delicate and difficult, and that his work might get people into trouble, Miss Manette is making it pretty clear that the prisoner is working as a secret agent.

The fact that she feels kindness and pity for the prisoner makes it hard for her to say anything that might damage his reputation, which probably explains why she didn't just come right out and say what she thought the prisoner's job was. In fact, she makes a point of calling the prisoner a "gentleman" at first, until she's directed to call him a prisoner, and she mentions how he was very kind and helpful to Miss Manette's father.