That day Phineas had actually not been with Ferb. Instead he was Isabella, who had invited him to a dance with the Fireside Girls, but she could only bring one friend. Phineas, being as oblivious as he his didn't even a note at this. He even ended up enjoying himself. After leaving, they returned to the Flynn-Fletcher household, since Phineas was hoping to still have time to build something. They first checked in the backyard. Empty. Not a sign of any green-haired kids, anywhere. They went inside and he was in neither the living room or kitchen. Which left one place: the boys' bedroom.