It's the course of nature that brings Tidus up to the deck. He feels a little more at peace when he's close to the water. He's up there nearly every night, sometimes recalling his time with his father and mother there, despite being at odds with their memories.

Auron's presence is never unusual. While he is not a constant, his sudden arrivals are and have long since stopped being true surprises. Auron is always there just the right amount of time he needs to be. It'd be stranger if Tidus got a proper greeting or some prior warning from him instead of always being something he happens upon as a silhouette or a voice in his ear.

Tidus can't allow for things to remain silent, given his brash nature, so he puts his hands on his hips and calls out. It doesn't matter that he's undoubtedly already been noticed, he'll announce his presence here without hesitation. It's a reminder to Auron and everyone that he exists.

"Don't you ever knock? Or use the door?"

It's not really an offended tone that he's using, just one sharp enough to center things in on himself.