Without a more specific story location in mind, the answer to this question could be several different situations. Additionally, "unhappy" could mean a wide variety of things. Passepartout's unhappiness could mean that he is sad, angry, frustrated, etc. I think the first really good possibility for this answer is when Passepartout admits that he is worried about his gas burner. Earlier in the book, he realized that he had left on his gas burner. Fogg told Passepartout that he would have to pay for the extra cost, and every day that the trip takes longer is more money out of his pocket.
"I was going to tell you there's one thing that worries me—my burner!"
"My gas-burner, which I forgot to turn off, and which is at this moment burning at my expense. I have calculated, monsieur, that I lose two shillings every four and twenty hours, exactly sixpence more than I earn; and you will understand that the longer our journey—"
The next solid example of Passepartout being unhappy occurs when he and Fogg discover that the railway they are on is not finished like the papers said it was. Passepartout is just about angry enough to throw a punch.
Passepartout would willingly have knocked the conductor down, and did not dare to look at his master.
A third example is when Fogg is forced to pay a huge fine for Passepartout's actions at the Hindu temple. Readers are told that he is "crestfallen" at having cost his master so much time and money.
Mr. Fogg, offering his arm to Aouda, then departed, followed by the crestfallen Passepartout.