...with that face Takumi's making, I think he's saying that for his sake as much as he is Karua's
What especially bothers me about this, looking back at it, is that Takumi has been narrating his life despite it being such a 'mundane time'. The way this kind of self-narration happens in the first two Danganronpa games is that it marks a significant event from the outset- being in awe at coming to a famous academy for the first time. The third Danganronpa game has the protagonist stuffed in a locker and spacing out of her existence when she talks to herself, which puts a highly dissociative spin on that 'meta self-talk'- and well it would, she was kidnapped and possibly drugged for all we know.
In the Hundred Line prologue, we are introduced to the complex's emergency siren through Takumi and for all we know about his life until that point, that's the first time he's ever heard it, the way he pauses in shock to take in the message. But it's not. He's heard it many times, so many in fact that you'd expect him to roll his eyes and move Karua to hiding if he was really that nonchalant. His narration then proceeds to explain the presence of the sirens as 'something the society has accepted as normal', which is... a bit strange for someone who has accepted the presence of them without any issues.
The way this could have reflected any day in Takumi's life gives me the impression he's dissociating on and off at any point in any day, but we don't 'see' it happening because of how he narrates 'to us'.