It's a very weird thing when Riku runs into another Ultra he knows in their human form. His senses are definitely more perceptive than a normal human's, but they're not quite as sharp as a normal Ultra's. But he knows what it feels like, now, when he sees someone who looks human but really isn't. And he most certainly knows what his closest friends feel like.

So when he sees someone who is clearly not Leito at this restaurant, he's going to stare, blink, and look around. Because he's sure he senses Zero, but he doesn't see anyone familiar.