Having trained ears was something Usopp always prided himself in. Although his reflexes weren't up to par, his sense of hearing was one in a million, a very heightened sense, and he was proud to say that he could track the smallest of sounds.
Living on the Going Merry was full of noise. Between Luffy's screams of joy, Nami's demands, Zoro and Sanji's fights, and Chopper's excited shrieks, there wasn't much to miss sound-wise, but after a while, his keen ear lessened.
Usopp curses himself every time that someone sneaks up on him. Hates that enemies can come from nowhere and hit him from behind, hates that he can't hear Luffy come running towards him before he's crashing against the wall, hates that Sanji can catch him off guard at any time of day.
He blames it on his daydreaming, not wanting to admit to himself that he's lost a skill he loved, a skill he was very much satisfied with.
One of Sanji's favorite things to do is get reactions out of people. He loves sneaking up on the sniper when Usopp least expects it, if only to see his eyes get three times bigger, he laughs every time.
For once since boarding the Going Merry, Usopp is ready for Sanji's sneaking. As twilight reaches, Sanji waits for the food to cool before he can serve it, Usopp's realized that this is usually when Sanji decides to get his laughs in.
Usopp waits for about a half an hour; listening for footsteps, an exhale, the shuffle of clothes; anything, but to no avail. He gets up off of the deck to walk inside and see if everyone is in the galley, when he hears a whistle from above him.
"Looking for me?" Sanji almost sings, swinging himself down from the crow's nest. Suddenly all of Sanji's sounds are very clear, and Usopp forgets for a minute why he was outside in the first place, Sanji's smile is cocky, he's stifling a laugh. Sanji's expression is enough to make Usopp's frustration dissolve.
Usopp admits to himself that maybe he doesn't mind Sanji sneaking up on him after all.