They May Be Invisible to Jon, But Not to Santa

"Zoe wasn't there. And neither was Athena," Jon said, coming back into his bedroom. "Are you _sure_ about that?" Santa asked. "I didn't see anyone," Jon said, then watched, stunned, as Santa abruptly propelled himself towards the door and grabbed something that Jon couldn't see. "Gotcha," Santa said. "Hey, let go!" "Zoe?!" Jon yelled. She was invisible? How was that possible?