Why does Atticus think Jem and Scout disobeyed him when he told them to stay in front of the Radley house?

Miss Maudie's house is on fire. Atticus tells his kids to stay by the Radley place. When Atticus returns, Scout has a blanket draped over her shoulders. He says, "Then whose blanket is that?" Atticus assumes they had moved. It was Boo Radley who had placed it on her during the excitement.