@Scmaxwell44 Well they hinted at it pretty strongly especially in the lastest chp though they've been building it up since the earlier chps. MC Hiyama messed up a previous case because he sympathised with a perpetrator. As a result, a civilian was stabbed during the incident, leading him to be disgraced by other detectives. Now he works as a police officer (demotion). Anyways, fast forward - He applied to resign a month before the perp is released.
Everything from chp 1-2 is in the present and Okazaki doesn't really tie that much into his back story, except giving readers an insight into what the old 'Hiyama' from 5 years ago was like. The puppy incident just reveals his kind/hard-working nature that has continued to persist - through his traumatic backstory has kind of made him put up a wall against other people and proved to be an obstacle in his relationship with Okazuki (you know, internal conflicts, the past threatening the present).
That's my take on it anyhow.