Sirius was too busy enjoying the attention and mayhem he was causing to notice Ian had taken it. He did however notice Ian walking away and that was his signal to run.

He barked once and darted away. In the confusion he was lost easily and people were more preoccupied either carrying about their business or picking up their wares.

When Sirius returned he was back in human form. He stepped to the side and discreetly cast a few spells to speed up the merchants' setting up shop again. Maybe he'd give buy something and give them a tip to make up for it.