I know what you did this summer
Maybe they're friendly neighbours who share a hobby?"They must be more than just neighbours."
Sort of, yes. Or rather, mind your own business in general. There's no reason to ever talk to your neighbors unless you have to. It didn't used to be that way, and I believe at one point Kaji comments that being friendly with your neighbor was like something from a previous era. I think in the chapter where he comes over to Shun's place and Neighbor-san returns some borrowed soy sauce.Is there some "You must ignore and/or hate your neighbour," mentality in Japan or something? They all act as if it's so weird to just be friendly with people who live around you.