Where the Smile Went, also known as The Whereabouts of the smile is Chapter 24 of Tsubasa: Reservoir Chronicles.
Syaoran sprints worriedly to Sakura, alarmed by Mokona's cry. Turns out, Sakura had only fallen asleep again (this was in part to one of Mokona's 108 Secret Techniques -- Super Dramatic Power). Fai reminds Syaoran that they've seen Sakura fall asleep every now and again, and that at least this time it didn't happen when they were in dire straits. He reminds Syaoran that he shouldn't be so scared or nervous all the time -- if he smiled or relaxed for a second, nobody would blame him for slacking on the job. He pointedly adds that someone may be happier for it.
Syaoran recalls the time when Sakura brought him to see the birds when they were children. It was the first time Syaoran had smiled at Sakura, who considered it a great personal accomplishment that she was able to make the ordinarily morose boy crack a grin.
At this moment, Sakura awakens -- and then suddenly runs to the lake, remembering that Syaoran is still underwater! Syaoran grabs her as she zooms to the lake and tells her that he's right there! She smiles back in relief. As the five of them take time to regroup, Fai tells Sakura that none of them have any idea what the journey holds for them, and that Sakura herself may be worried about retrieving her feathers, but they should try to make the trip as pleasant as possible. Sakura nods, and bows, trusting her fate and her memories to these four strangers. Syaoran can only look in admiration: even without her memories, Sakura's optimism and hope remain intact.
Mokona and Fai turn to Syaoran and ask him about his underwater exploration. Syaoran excitedly relays the information that he found a city underwater. With this, they deduce that the power that Mokona had detected wasn't coming from one of Sakura's feathers. Kurogane complains that this world was nothing but a waste of time, but Sakura observes that Syaoran looked like he had fun seeing the underwater city. He grins back, saying that it's always fun when he sees something wonderful with his own eyes.
Mokona prepares them for their next destination, and for a second, Syaoran glances sideways at Sakura. Sakura turns and smiles warmly at him, and he recalls how he used to love seeing her smile. Just looking at it gives him warmth in his chest. All he wanted was to see her smile.