Judging by the way he looks up stream, I think the simple insinuation is that "THAT" simply refers to the death of the child in general (and the possible subsequent haunting that might come with it). He picks of the lost item not giving it much though but upon realizing that the items might belong to someone deceased he sets it back down and offers a prayer. Unfortunately it was too late and he got marked, but fortunately his haunting was benevolent(?).A rare heartfelt chapter. Maybe. <3
As far as the "What if...they're here because of that?", the year is 2012, a year after the 2011 earthquake and tsunami. Sapporo isn't quite on the East coast, though.