Is the hint supposed to be something in this chapter, or something that'll happen in one of the following chapters? If it's this chapter, then my instinct would be that the car explosion is meant to be the hint. Either that, or him ominously walking backwards on the roof.
If the explosion is the hint, though, then how is that supposed to help him? Is there anything about it that implies anything about the culprit? Is it the fact that he was able to acquire the grenade in the first place? The fact that he was able to subdue his mom? The details of how the trap was set up in the first place?
Or is the hint from earlier, before he mentioned it? Like, was the fact that he even came here the "hint?" If so, then was him panting and crying while looking at Nagisa the hint? Are the bike tricks the hint? Is the fact that he was able to get into the apartment complex the hint?