The trace is not quite precise. Harry got an official warning for a spell Dobby performed, which even confirms that. There must be adult wizards on the train, and Diagon Ally is swamped with it. Anyone could have performed the spell and the trace would react. When Harry used the Patronus to save Dudly he was the only wizard around (since Figg was a squib), so then only he could have done it with all trouble that would come with it. This was also clarified by Rowling herself.
