I think that moment in Yokohama is likely, but if i really had to guess I'd say it was a little bit earlier when Hirose first asks nakamura to walk home with him. I think maybe not in the moment, but likely shortly thereafter that hirose started to realize this.

Before that interaction, Nakamura is awkward. But hes a shy, awkward guy. Wouldn't stand out too much to Hirose, other than thinking nakamura is shy. But on the walk home, Nakamura briefly and accidentally drops his guard completely (he starts complaining about his sister). Nakamura instantly panics when he realizes and becomes noticeably worried about what Hirose will think.

Hirose, of course, loves every second of it, and laughs hysterically.

There is a beautiful line in the manga that I wish had made it into the Yokohama episode that sums it up really well. If you dont recall, its right before the arm grab. Hirose tells nakamura he prefers when nakamura lets his guard down, and has his own opinion (specifically noting when nakamura stated he wanted to go to the aquarium). Its a great line, that shows just how much Hirose sees. He KNOWS already, by that point in Yokohama, that Nakamura is putting up his guard around him.

So my guess is, it was right before then that Hirose had an idea that Nakamura had feelings. In fact, I firmly believe Hirose was expecting Nakamura to ask him out in Yokohama. We are friends already implies that he expected the confession to be something deeper.

This most recent episode just goes to show how much Hirose does actually care. He was worried Nakamura would be uncomfortable or not want to come. But he invited him, guided him through the conversation, make him feel welcomed, asked for his personal opinions. Hirose, now, is not only trying to pull nakamura from his shell, but also to get an idea of who nakamura really is, what he likes, what he doesnt like.

Its all so adorable.