Luke had just been at another shop, asking about the DVDs for Sarah Jane. She hadn't told him all the details, he was pretty sure, but it was a safe enough mission for him to do. Go around to a bunch of shops and try to find movies.
He knew Sarah Jane wasn't far - and as soon as he left that shop, he thought he saw her car parked a bit down the street. He headed over, stepping in and seeing her immediately. "Mum," he greeted her, walking over and standing by her side. "That shop on the corner didn't have them, either," he reported, with an apologetic look.
Then, he smiled politely to the shop's employee. "Hello."