Jabez Wilson tells Holmes and Watson that when he found the notice that the Red-Headed League had been dissolved, he immediately went to the landlord, who told him that the office had been rented by a man who called himself William Morris, not Duncan Ross, and that Morris had moved out yesterday. The landlord was able to provide Morris's new address: 17 King Edward Street, near St. Paul's. When Wilson goes there, however, he finds that:
"...it was a manufactory of artificial knee-caps, and no one in it had ever heard of either Mr. William Morris or Mr. Duncan Ross.”
This is what makes Wilson believe that the whole business of the Red-Headed League was what he calls a "prank." It also prompts him to consult Sherlock Holmes. Wilson has little hope of regaining his job, but he feels hurt, angry, and humiliated and, as he says:
"I want to find out about them, and who they are, and what their object was in playing this prank—if it was a prank—upon me. It was a pretty expensive joke for them, for it cost them two and thirty pounds.”
Holmes only accepts the case because he feels certain that there is something much more serious behind the Red-Headed League than a mere prank. He also tells Watson later that he felt that time was of the essence. The fact that the League has been dissolved that day meant that the crooks no longer needed to keep Wilson out of the way and were ready to strike. But Holmes has no idea what sort of crime was planned until he and Watson visit the Saxe-Coburg Square, where Wilson has his pawnshop. From what Holmes observes there he deduces that the crooks are digging a tunnel from the pawnshop to a nearby bank and intend to loot it.